An artificial solid electrolyte interphase (SEI) consisting of a lithium-ion conducting material enables the use of a LiNi0.5Mn 1.5O4 cathode with conventional carbonate electrolytes. The coulombic loss and electrolyte oxidation are largely remedied by the nanometer-scale artificial SEI. The thickness of the artificial SEI is optimized by balancing the protection and additional resistance. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.