Tunable multi-doped carbon nanofiber air cathodes based on a poly(ionic liquid) for sodium oxygen batteries with diglyme/ionic liquid-based hybrid electrolytes

A series of multi-doped hierarchical fibrous network structures have been synthesized using a facile electrospinning method and subsequent pyrolysis, resulting in binder-free self-standing cathodes for Na-O₂ batteries. Such binder-free electrodes are important to avoid the known issue of degradation during battery operation. A poly(ionic liquid) (poly(diallyldimethylammonium) bis(fluorosulfonyl)imide (PDADMA FSI)), a source of multidoped (S, N and F configurations) carbon nanofibers, and polyacrylonitrile (PAN) were used as co-precursors. PDADMA FSI plays an essential role in tuning the morphology (fiber diameter, specific surface area and pore size) of the carbon nanofibers providing a strategy to control electrocatalytic activity. In fact, increasing the amount of PDADMA FSI in the precursor mixture led to larger specific surface area and smaller fiber diameter, enhancing the discharge capacity of the battery. The multi-doping and the synergetic effect of pyridinic-N configurations, ionic C-F bonds and thiophene S bonds (C-S-C) in the air cathode enhanced the oxygen reduction process, and hence the discharge capacity (0.35 mA h cm⁻² at 0.6 mA cm⁻²). The air cathodes have been assessed for Na-O₂ batteries in a series of electrolytes (e.g., ionic liquids, organic solvents and a hybrid electrolyte) presenting excellent long-term cycling (157 cycles), the highest reported for N-doped carbon-based air cathodes.