context: In situ GIWAXS was used to probe the crystallinity and crystallographic texture of the initial SEI layer formed on copper in 100 ppm added HF electrolyte. The initial SEI, formed by galvanostatically bringing the potential down to 0 V vs. Li/Li+ by cycling at 0.5 mA cm−2, was found to contain crystalline LiF particles as evidenced by the small broad peak at 2.71 Å−1 in the inset of Fig. 8a, corresponding to the (111) crystallographic plane. These LiF crystallites exhibit (111) texturing as shown by the peaks at ±70.5° in the I(χ) analysis (Fig. 8b and S5†), which is reasonable given the face centered cubic rock salt structure of LiF. Scherrer analysis of the LiF (111) peak width from the I(q) data indicates a crystallite size on the order of ∼5 nm, slightly larger than the SEI thickness calculated from both charged passed and XPS depth profiling, suggesting LiF does not form a continuous film. The other SEI components are either amorphous or too thin to characterize with GIWAXS. No crystallographic texture is evident in the SEI formed on copper from an electrolyte without added HF (Fig. S4†). The texturing may only occur in electrolyte with added HF because LiF is formed at higher potential vs. Li/Li+ due to selective HF reduction, and LiF is the only solid product of this reaction. Therefore, the reduction process does not have to compete for reactants (lithium ions, fluorine atoms, electrons) or physical space on the copper surface, resulting in more facile LiF deposition homogenously distributed across the working electrode. In the case without HF, multiple reduction processes (e.g. PF6−, solvent) which yield multiple solid products (e.g. LiF, Li2O, Li2CO3, organic species) all occur simultaneously at the applied scan rates, resulting in more random LiF formation.
answer: HF 