Tetracene crystals as promising anode material for alkali metal ion batteries

Using first-principles calculations we study the energetics of alkali metal (AM) atom intercalation into bulk tetracene crystals. We show that, contrary to the adsorption of Li and Na atoms on isolated tetracene molecules, the intercalation of these and other (K, Rb, Cs) AM atoms into bulk tetracene crystals is energetically favorable (with respect to forming an infinite AM crystal) in a wide range of AM concentrations and that the intercalation energy is noticeably lower than that for intercalation into graphite, a material used today in anodes of AM batteries. In case of Li, there is no swelling of the intercalated crystals, and for Na the increase in crystal volume is less than 10%, which makes crystalline tetracene attractive from the viewpoint of energy storage, as the capacity exceeds the theoretical capacity of graphite. We further assess diffusion barriers of AM atoms, which for Li and Na proved to be below 0.5 eV, indicating a high diffusivity of these atoms already at room temperature. We also study the effects of intercalation on the electronic properties of the system, and show that several bands can be filled upon intercalation, so that the system exhibits semiconducting–metallic–semiconducting behavior when AM atom concentration increases. Our results shed light on the perspective of using AM atom intercalation into tetracene crystals for energy storage and tuning the electronic properties of this system.