Hongjiao Nie

Hierarchical Micron-Sized Mesoporous/Macroporous Graphene with Well-Tuned Surface Oxygen Chemistry for High Capacity and Cycling Stability Li–O2 Battery

Nonaqueous Li-O2 battery is recognized as one of the most promising energy storage devices for electric vehicles due to its super-high energy density. At present, carbon or catalyst-supporting carbon materials are widely used for cathode materials of Li-O2 battery. However, the unique electrode reaction and complex side reactions lead to numerous hurdles that have to be overcome. The pore blocking caused by the solid products and the byproducts generated from the side reactions severely limit the capacity performance and cycling stability. Thus, there is a great need to develop carbon materials with optimized pore structure and tunable surface chemistry to meet the special requirement of Li-O2 battery. Here, we propose a strategy of vacuum-promoted thermal expansion to fabricate one micron-sized graphene matrix with a hierarchical meso-/macroporous structure, combining with a following deoxygenation treatment to adjust the surface chemistry by reducing the amount of oxygen and selectively removing partial unstable groups. The as-made graphene demonstrates dramatically tailored pore characteristics and a well-tuned surface chemical environment. When applied in Li-O2 battery as cathode, it exhibits an outstanding capacity up to 19 800 mA h g(-1) and is capable of enduring over 50 cycles with a curtaining capacity of 1000 mA h g(-1) at a current density of 1000 mA g(-1). This will provide a novel pathway for the design of cathodes for Li-O2 battery.

Free-Standing Thin Webs of Activated Carbon Nanofibers by Electrospinning for Rechargeable Li–O2 Batteries

Free-standing activated carbon nanofibers (ACNF) were prepared through electrospinning combining with CO2 activation and then used for nonaqueous Li-O2 battery cathodes. As-prepared ACNF based cathode was loosely packed with carbon nanofibers complicatedly overlapped. Owing to some micrometer-sized pores between individual nanofibers, relatively high permeability of O2 across the cathode becomes feasible. Meanwhile, the mesopores introduced by CO2 activation act as additional nucleation sites for Li2O2 formation, leading to an increase in the density of Li2O2 particles along with a size decrease of the individual particles, and therefore, flake-like Li2O2 are preferentially formed. In addition, the free-standing structure of ACNF cathode eliminates the side reactions about PVDF. As a result, the Li-O2 batteries with ACNF cathodes showed increased discharge capacities, reduced overpotentials, and longer cycle life in the case of full discharge and charge operation. This provides a novel pathway for the design of cathodes for Li-O2 battery.

Iridium incorporated into deoxygenated hierarchical graphene as a high-performance cathode for rechargeable Li–O2 batteries

A novel Li-O-2 cathode was designed with a nanocrystal iridium catalyst functionalized on the purposely deoxygenated surfaces of hierarchical graphene. Due to the synergistic effect between the ORR/OER activity and deoxygenated porous supporter, this cathode exhibited excellent battery performance, cycling 150 times with a limited capacity of 1000 mA h g(-1) at a current density of 2000 mA g(-1).

Synthesis of a meso–macro hierarchical porous carbon material for improvement of O2 diffusivity in Li–O2 batteries

Meso–macro hierarchical porous carbon (HPC) is prepared and used as a cathode material in Li–O2 batteries. The O2 diffusivity has been largely improved due to the unblocked macropores. As a result, a better pore utilization and extremely high discharge capacity is achieved.