S1 Electric Potential Gradient at the Buried Interface between Lithium-Ion Battery Electrodes and the SEI Observed Using Photoelectron Spectroscopy Julia Maibach a *, Fredrik Lindgren a , Henrik Eriksson a , Kristina Edström a , Maria Hahlin b a) Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE 751 21 Uppsala, Sweden b) Department of Physics and Astronomy, Uppsala University, Box 516, SE 751 21 Uppsala, Sweden Corresponding Author *Dr. Julia Maibach, julia.maibach@kemi.uu.se