A high-energy lithium ion capacitor that has Li2MoO3 as an alternative lithium source instead of metallic lithium has been proposed. For further improvement, we suggest Li2RuO3 as a new additive to improve the energy density in the positive electrode. The choice of Li2RuO3 is made based on its highly reversible characteristics for Li+ insertion and extraction and its structural stability in the operating voltage window of advanced lithium ion capacitors. The electrochemical and structural properties of Li2RuO3 have been thoroughly investigated to demonstrate its potential use in lithium ion capacitors. The high reversibility of Li2RuO3 and the metallic feature of Li2-xRuO3 may be responsible for improvements in the volumetric energy density and safety. This versatile approach may yield higher energy density without significant power loss in lithium ion capacitors. © 2013 American Chemical Society.