context: The chemical composition and structure of the SEI after 1.0 mA h cm−2 deposition at 0.2 mA cm−2, 0.5 mA cm−2 and 1.0 mA cm−2 were further probed by XPS depth profiling. The XPS depth profiles of the N 1s, Li 1s, F 1s, C 1s, and O 1s spectra are shown in Fig. 6a–i and S13–S15.† At 0.2 mA cm−2 (Fig. 6a–c), the major inorganic components comprising Li3N and LiF were distributed homogeneously in the depth direction of the SEI. Li3N, as the reduction product of LiNO3, is a well-known Li ion conductor with a conductivity of 10−3 S cm−1 at room temperature; LiF, either as an electrolyte additive or SEI component, can improve the diffusion of lithium ions at the electrode surface. The mixed Li3N and LiF layer can facilitate rapid lithium ion diffusion, stabilize the SEI and provide uniform electrodeposition. For the high current densities, especially 1.0 mA cm−2 (Fig. 6g–i), the SEI lacked the inorganic components Li3N and LiF; instead, organic ROLi species were observed. The poor ionic conductivity of ROLi impeded sufficient transport of Li ions towards the electrode surface at high current densities and induced preferential deposition. The very different SEI composition resulting from the repeated SEI breakdown and repair processes also reinforced the rupturing and re-forming cycles of the SEI. In addition, the XPS chemical composition results are in accordance with the mechanical properties obtained by AFM force probing. The SEI formed at a low current density contained a higher content of inorganic components and greater mechanical strength, while the SEI formed at a high current density contained a mixed organic–inorganic layer and was relatively soft (Fig. 6j–l).