Lightweight, free-standing 3D interconnected carbon nanotube foam as a flexible sulfur host for high performance lithium-sulfur battery cathodes

Abstract

The still hindered practical application of lithium-sulfur (Li-S) batteries with a high theoretical energy density of 2.6 kWh kg−1 can only be feasible by a simple and scaling-up fabrication of highly stable sulfur-based cathodes. Herein, a free-standing, mechanically flexible, binder-free 3D interconnected carbon nanotube ‘foam’ (CNTF) is prepared by a single-step facile method and used as a sulfur host in Li-S batteries. For the first time, such a simple method has been adopted for the preparation of free-standing CNT scaffolds for use in Li-S cells, as our method is free from the widely reported solvent-based techniques such as vacuum infiltration of CNTs to obtain free-standing forms but requires further purification and/or drying. A high-areal sulfur loading of 7.1 mgS cm−2, accounting to a total electrode mass of 10.9 mgelectrode cm−2, with yet high electrochemical sulfur utilization of 72% is achievable by the foam-like CNT structure. Reversible areal capacities of up to 9 mAh cm−2 at extremely low electrode weight (800 mAh gelectrode−1) and specific capacities up to 1378 mAh gS−1 are demonstrated. The interconnected porous network acts as a reservoir for trapping soluble lithium polysulfide compounds and greatly improves the sulfur reutilization. The lightweight CNT scaffold further provides enduring electrical contact with the sulfur species, resulting in excellent cycling stability and a potentially high gravimetric energy density desirable for automobiles and aerospace applications. The CNTF/sulfur composite cathode exhibits better rate performance and cycling stability than most of the recently reported CNT-based cathode materials for Li-S batteries.