Binder-free graphene foams for O₂ electrodes of Li-O₂ batteries

We report a novel method to prepare bind-free graphene foams as O ₂ electrodes for Li-O₂ batteries. The graphene foams are synthesized by electrochemical leavening of the graphite papers, followed by annealing in inert gas to control the amount of structural defects in the graphene foams. It was found that the structural defects were detrimental to the processes of the ORR and OER in Li-O₂ batteries. The round-trip efficiencies and the cycling stabilities of the graphene foams were undermined by the structural defects. For example, the as-prepared graphene foam with a high defect level (I_D/I_G = 0.71) depicted a round-trip efficiency of only 0.51 and a 20^th-cycle discharge capacity of only 340 mA h g^-1 at a current density of 100 mA g^-1. By contrast, the graphene foam electrode annealed at 800 °C with I _D/I_G = 0.07 delivered a round-trip efficiency of up to 80% with a stable discharge voltage at ∼2.8 V and a stable charge voltage below 3.8 V for 20 cycles. According to the analysis on the electrodes after 20 cycles, the structural defects led to the quickened decay of the graphene foams and boosted the formation of side products.