Controlling the Three-Phase Boundary in Na-Oxygen Batteries: The Synergy of Carbon Nanofibers and Ionic Liquid

A series of electrospun binder-free carbon nanofiber (CNF) mats have been studied as air cathodes for Na-oxygen batteries using a pyrrolidinium-based electrolyte and compared with the commercial air cathode Toray 090. A tenfold increase in the discharge capacity is attained when using CNFs in comparison with Toray 090, affording a discharge capacity of 1.53 mAh cm(-2) at a high discharge rate of 0.63 mA cm(-2). The good specific discharge and charge capacities of these CNFs are determined by the void space and the highly accessible surface of the carbon fiber. Furthermore, a threefold increase has been attained in terms of specific capacity by controlling the flooding of the air cathode and hence the location of the three-phase boundary within the CNF mat. The enhancement in performance has been correlated to the morphology, composition, distribution, and location of the discharge products. Sodium superoxide and peroxide were identified as the discharge products and, more importantly, the common side reaction discharge products, which are known to be detrimental to battery performance (including sodium fluoride, sodium hydroxide, and formate), were not observed, exemplifying the stability of the pyrrolidinium-based electrolyte and these binder-free CNF air cathodes.