Nitrogen/Phosphorus/Fluorine Heteroatoms Codoped Carbon Nanotube Networks as Free-Standing Cathode for Rechargeable Li-CO2 Batteries

Rechargeable lithium–carbon dioxide (Li-CO2) batteries have garnered global interest for their CO2 capture potential and exceptionally high energy density. However, the sluggish kinetics and elevated charging potential induced by the wide band gap insulator lithium carbonate (Li2CO3) underscore the critical need to investigate cathode catalysts that can facilitate the decomposition of Li2CO3 and lower charging potential. Heteroatom doping plays an important role in regulating the catalytic activity of carbon-based metal-free catalysts. Herein, nitrogen/phosphorus/fluorine heteroatoms codoped carbon nanotubes (NPF-CNTs) are synthesized in one step by ammonium hexafluorophosphate (NH4PF6) assisted route. The three-dimensional interconnected structures of the free-standing NPF-CNT membrane cathode are fabricated by a simple vacuum filtration, which provides abundant active sites and facilitates charge transfer during the CO2 electrochemical reaction. The batteries with NPF-CNT cathodes achieve a high discharge area capacity (2.86 mAh cm–2) and a low overpotential and the excellent cycle performance (900 cycles at 100 μA cm–2, approaching 1800 h). The good cycling stability at 200 μA cm–2 is more prominent for the Li-CO2 batteries. These results demonstrate that the NPF-CNTs are crucial in enhancing the kinetics of the CO2 reduction and evolution reactions, thus significantly improving electrochemical performance.