A comparative computational study of lithium and sodium insertion into van der Waals and covalent tetracyanoethylene (TCNE)-based crystals as promising materials for organic lithium and sodium ion batteries

Abstract

We present a comparative ab initio study of Li and Na insertion into molecular (van der Waals) crystals of TCNE (tetracyanoethylene) as well as in covalent Li/Na-TCNE crystals. We confirm the structure of the previously synthesized (covalent) Li-TCNE crystal and predict the existence of its Na-TCNE analogue. In the molecular crystals, we compute the maximum voltages to be 3.5 V for Li and 3.3 V for Na, with theoretical capacities of 1247 mA h g⁻¹ for Li and 416 mA h g⁻¹ for Na. In the covalent crystals, the maximum voltages are 2.2 V for Li and 2.7 V for Na, and theoretical capacities are 394 mA h g⁻¹ for Li and 176 mA h g⁻¹ for Na. Significantly, up to a capacity of 416 mA h g⁻¹ for both Li and Na in the molecular crystal and 197 mA h g⁻¹ for Li and 176 mA h g⁻¹ for Na in the covalent crystal, the insertion of Li and Na would not lead to reactions with common electrolytes. We show that volumetric capacities of organic electrodes need not be low compared to their inorganic counterparts, contrary to popular belief: the molecular TCNE crystal has been computed to achieve the values of 1845 mA h cm⁻³ for Li and 615 mA h cm⁻³ for Na, respectively. Tetracyanoethylene-based molecular and covalent crystals could therefore become efficient organic cathode and anode materials for Li and Na ion batteries.