Li−air batteries represent cutting edge electrochemical energy storage devices, but their practical applications have been precluded by the high cathode cost, the low discharge/charge efficiency, and/or the short battery lifetime. Here, we developed a low-cost, but very efficient, air electrode from porous nitrogen-doped holey graphene for rechargeable nonaqueous Li−O 2 cells. The resultant Li−O 2 cell can deliver a high round-trip efficiency (85%) and a long cycling life (>100 cycles) under controlled discharge/charge depths or a high capacity of 17 000 mAh/g under the full discharge/charge condition, superior to most other carbonaceous air cathodes. The observed superb performance for the air electrode based on the nitrogen-doped holey graphene can be attributed to its efficient metal-free catalytic activity and threedimensional mass transport pathway. Therefore, this work represents a new approach to low-cost, efficient, metal-free, binder-free, and hierarchically porous air electrodes useful for energy conversion and storage from N-doped holey graphene.

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Having a large surface area, high mechanical strength, excellent electrical and thermal properties, graphene is attractive for a wide range of potential applications, including energy conversion and storage. To realize commercial reality of graphene-based energy devices, it is highly desirable to produce highquality graphene at a low cost and large scale. In this review, we will give an overview on large scale production of edge-selectively functionalized graphene nanoplatelets by mechanochemical ball-milling and their uses for energy conversion and storage.

Graphene network (GN) was synthesized by a two-step chemical vapour deposition (CVD) method, involving the thermal annealing sputter-coated Cu film to form a Cu network by annealing for CVD deposition of graphene onto the Cu network catalyst. The resultant graphene network was transferred onto a flexible and transparent polymer (e.g., PDMS) substrate while maintaining its porous structure and integrated interconnection, providing both good optical transparency (e.g., transmittance of 86% at 550 nm wavelength) and mechanical flexibility. Flexible and transparent all-solid-state supercapacitors based on the newly-developed graphene network were fabricated to exhibit an area specific capacitance of 4.2 mF cm À2 at a discharge current of 0.1 mA with a high optical transparency (transmittance of 84%), which outperforms devices based on uniform multi-layer graphene sheet.

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