A Metal-Free, Free-Standing, Macroporous Graphene@g-C3N4 Composite Air Electrode for High-Energy Lithium Oxygen Batteries

The nonaqueous lithium oxygen battery is a promising candidate as a next-generation energy storage system because of its potentially high energy density (up to 2-3 kW kg(-1)), exceeding that of any other existing energy storage system for storing sustainable and clean energy to reduce greenhouse gas emissions and the consumption of nonrenewable fossil fuels. To achieve high energy density, long cycling stability, and low cost, the air electrode structure and the electrocatalysts play important roles. Here, a metal-free, free-standing macroporous graphene@graphitic carbon nitride (g-C3N4) composite air cathode is first reported, in which the g-C3N4 nanosheets can act as efficient electrocatalysts, and the macroporous graphene nanosheets can provide space for Li2O2 to deposit and also promote the electron transfer. The electrochemical results on the graphene@g-C3N4 composite air electrode show a 0.48 V lower charging plateau and a 0.13 V higher discharging plateau than those of pure graphene air electrode, with a discharge capacity of nearly 17300 mA h g((composite))(-1). Excellent cycling performance, with terminal voltage higher than 2.4 V after 105 cycles at 1000 mA h g((composite))(-1) capacity, can also be achieved. Therefore, this hybrid material is a promising candidate for use as a high energy, long-cycle-life, and low-cost cathode material for lithium oxygen batteries.