Relation between SEI thickness and Young modulus: (a)–(d) for samples 2–5 (blue arrow: discharged to 0.05 V, 0.005 V, 0.005 V and kept at 0.005 V for 48 h, charged to 2 V; EC-based electrolyte, RT), (e)–(i) for samples 6–10 (olive arrow: discharged to 0.2 V 0.05 V, 0.005 V, 0.005 V and kept at 0.005 V for 48 h, charged to 2 V; FEC-based electrolyte, RT), (j)–(k) for samples 11–12 (red arrow, discharged to 0.005 V and kept at 0.005 V for 48 h, charged to 2 V; FEC-based electrolyte, 55 °C). Black square is for single-layered SEI films; red square and olive triangle are for the outer and inner layers of double-layered SEI films; blue square, violet triangle, and magenta pentagon are for the outer, middle, and inner layers of triple-layered SEI films, respectively.