  We present a comparative ab initio study of Li and Na insertion in molecular
(van der Waals) crystals of TCNE (tetracyanoethylene) as well as in covalent
Li/Na-TCNE crystals. We confirm the structure of previously synthesized
(covalent) Li-TCNE crystal as well as predict the existence of its Na-TCNE
analogue. In the molecular/covalent TCNE crystals, insertion sites are
identified with the binding energy of Li and Na up to 2.7/1.8 and 2.6/1.8 eV
stronger than Li and Na cohesive energy, respectively, in dilute
concentrations. Up to 5.5/2.5 and 3/2 Li and Na atoms per TCNE unit can be
inserted in the molecular/covalent crystals, respectively, while preserving the
structure, with maximum voltages, respectively, 3.5/2.2 and 3.3/2.7 V.
Significantly, up to capacity of 418 mAh g-1 for both Li and Na in the
molecular crystal and 198 mAh g-1 for Li and 177 mAh g-1 for Na in the covalent
crystal, the insertion of Li and Na would not lead to reactions with common
electrolytes. Tetracyanoethylene-based molecular and covalent crystals could
therefore become efficient organic cathode and anode materials for Li and Na
ion batteries.
