Co-Ni Alloy Encapsulated by N-doped Graphene as a Cathode Catalyst for Rechargeable Hybrid Li-Air Batteries

A hybrid Li-air battery uses a protected lithium anode and a porous air cathode in aqueous electrolyte, basing on a 4-e ORR/OER reaction. It avoids the insoluble and insulating Li2O2 product in a typical non-aqueous Li-air battery and it owns unique advantages. A bifunctional cathode catalyst is crucial to the battery performance. Here, we synthesize an ultrathin N-doped graphene encapsulated nanosphere Co-Ni alloy (Co-Ni@NG). It has hierarchical architecture consisting of ultrathin graphene shells (only 1-3 layers) that encapsulate a uniform Co-Ni nanoalloy, showing high activity, stability and lower overpotential between ORR (onset potential of 0.95 V) and OER (onset potential of 1.5 V). A discharge/charge voltage gap of 0.55 V at a currnet density of 1.4 mA cm-2, which is much smaller compared with commercial Pt/C catalyst. It delivered an energy density of 3158 Wh kg-1 and a power density as high as 134.2 W m-2 at a current density of 7 mA cm-2. The graphene shells protect the alloy catalyst and impro...