Abstract A new structural phase is discovered for M 2 CO 2 MXenes with M = Sc, Y, La, Lu, Tm, and Ho. The hexagonal carbon layer sandwiched between M atoms, typical for MXenes, is transformed into C 3 trimers with anionic electrons localized in quasi zero‐dimensional lattice spaces in‐between the C 3 units, so the systems can be described as [M 6 C 3 O 6 ] +II : 2e − electrides. The systems are readily ionized into [M 6 C 3 O 6 ] +II with very low ionization energy via an anti‐doping mechanism. It is shown that this new structure of Sc 2 CO 2 can bind multiple lithium atoms, with low migration barriers. The findings indicate that these M 2 CO 2 MXenes with unusual carbon trimers are a new family of 2D electride insulators with the potential for charge storage applications, thermal field emission, and as anode material in lithium batteries.