In past decades, there have been great achievements on theoretical prediction and experimental fabrication of two-dimensional transition metal carbides. In this work, we report a hitherto unknown atomic-thin planar-structured transition metal carbide sheet denoted as MC6 (M = Cu, Ag, Au) via a structure-swarm intelligence algorithm. The proposed 2D crystals are both statically and dynamically stable, and could maintain their structure integrity at temperatures up to 1500 K. The calculated specific capacities of lithium-ion batteries employing such materials reach 1188 mA h g-1, which is 26.38% higher than previously reported highest MXene material with Mn+1Xn atomic configuration. Our results present the potential application of MC6 in Li-ion batteries.