Li-O₂ battery has got much attention as a beyond Li-ion battery (LIB), due to its higher energy density and exceeding specific capacity than those of commercial LIB. Though it has a promising potential for large-scale energy storage device, Li-O₂ battery still has several problems to be overcome in aspects of poor rate capability and limited stability. Especially, the oxygen electrode is one of the most closely related component to the performance of Li-O₂ battery since it is the actual site for formation and decomposition of discharge products. Thus, in order to achieve the Li-O₂ battery with improved performance, the oxygen electrode should provide favorable structure for not only discharge product storage, but also facile transfer of reactants such as Li ion and O₂. Moreover, bicatalytic activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is also significant on oxygen electrode by determining the morphology of discharge product. Furthermore, the oxygen electrode has to be chemically and electrochemically stable as well for extended durability of Li-O₂ battery.

In this regards, we develop a binder free oxygen electrode composed of nitrogen-doped hollow carbon nanofiber, which is derived from nitrogen-containing polymer. Each carbon nanofiber has an empty space inside the fiber even after the heat treatment at high temperature of 900 °C, and it can facilitate the mass transfer of reactants. Due to this unique structure and high nitrogen content of 4.5 at. %, nitrogen-doped hollow carbon nanofiber indicates the lower ORR overpotential and larger discharge capacity than commercial carbon material, Super P, as shown in Fig. 1. Interestingly, it shows the reduced ORR/OER overpotential which is lower than 1 V with the curtaining capacity of 500 mAh g^-1_carbon. The notably reduced overpotential might be attributable to the synergetic effect of nitrogen doping, unique structure of hollow carbon nanofiber and absence of binder by promoting the catalytic activity, facilitating the mass transfer and suppressing the side reactions, respectively. The effects of heat treatment temperature and metal incorporation are also investigated and optimized. As for the result, it can be demonstrated that the binder-free nitrogen-doped hollow carbon nanofiber is an enhanced oxygen electrode of Li-O₂ battery.