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

TL;DR

This study compares lithium and sodium insertion in van der Waals and covalent TCNE crystals, revealing their potential as organic electrode materials for lithium and sodium-ion batteries based on their structural stability and capacity.

Contribution

It provides the first detailed ab initio comparison of Li and Na insertion in molecular and covalent TCNE crystals, predicting their electrochemical properties and capacity.

Findings

Li and Na can be inserted with high binding energies.

Maximum capacities of 418 mAh/g for both Li and Na in molecular crystals.

Insertion does not react with common electrolytes at high capacities.

Abstract

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.