context: To study the electrolyte reduction processes resulting in SEI formation, cyclic voltammetry (CV) was performed systematically as a function of added HF concentration (as-received, +10, +100, and +500 ppm) and voltage sweep rate. Fig. 5 shows a series of CV scans measured at 5 mV s−1 in electrolytes with varying concentrations of HF. The peak at ∼2 V, indicated by the dashed line, results from the electrocatalytic reduction of HF to form LiF (reaction (2)). The additional peak at higher potential in the as-received electrolyte may be attributed to either PF6− or POF3 reduction. The slight shift in peak position to higher potentials (from 1.9 to 2 V) with increasing HF concentration is consistent with the report by Strmcnik et al. Solutions with more HF exhibit higher peak currents and increased capacity going into SEI formation. If a uniform film of LiF alone were fully passivating the copper surface, this would be achieved at a specific capacity as further HF reduction would be prevented by the passivation, and increased HF in the electrolyte would not lead to increased SEI capacity. However, this data suggests that LiF alone does not passivate the copper surface at these HF concentrations and scan rates because increased HF concentration in the electrolyte results in a corresponding increase in the capacity of the HF reduction peak. Passivating LiF films can be formed under different conditions, however, namely at slower scan rates and on smoother single-crystalline substrates. We rationalize that HF reduction is limited by the availability of HF at this interface, which in turn determines the amount of LiF that is formed in the initial SEI.