| Lithium ion batteries are the most advanced power sources for portable electronic devices, hybrid electric vehicles and electric vehicles in a wide range of consumer due to the unique advantages, such as high energy density, long cycle life, and environmental benignity. Graphite is currently taken for standard anode material for LIBs. However, the relatively low theoretical capacity (372mAh/g) and the safety concern impeded its application in high-energy LIBs. Therefore, new electrode materials should be developed to meet the requirements in energy and power densities, safety and cycling stability.As an anode material for LIBs, NiO has high safety, environmental benignity, low cost, and outstanding theoretical capacity (718mAh/g). However, it still suffers from rate performance and poor cycle stability resulted by its low ionic and electronic conductivity, the large volume expansion and resulting electrode pulverization. Hollow or porous materials have numerous benefits for LIBs, such as higher rate capabilities, better cycle lives and sometimes greater gravimetric capacities at a given rate compared to nonporous bulk materials because they can lead to fast ion or electron transfer and sufficient contact interface between active materials and electrolyte. Moreover, graphene has been used to form hybrid materials with transition metal oxides for improving the energy storage performance because of its excellent electronic conductivity, high theoretical surface area, and good mechanical properties. In this paper, porous NiO hollow microspheres were prepared by ultrasonic deposition or amino acid assisted hydrothermal method and NiO-wrapped graphene composite was prepared by solution-based method under reflux conditions and subsequent annealing. The main contents and results are as following:1.Novel porous NiO hollow microspheres are fabricated by a facile two-step method involving the ultrasound-assisted synthesis of nickel oxalate precursors and subsequent thermal annealing in air. The as-prepared NiO microspheres show porous hollow characteristics, and each microsphere is composed of loosely packed nanoparticles with diameters around30-80nm. When evaluated as anode material for lithium ion battery, the porous NiO hollow microspheres showed outstanding electrochemical performance with high lithium storage capacity, satisfactory cyclability and rate capacity. The reversible capacity of the NiO microspheres retained380mAh/g after30cycles at200mA/g. Even when cycled at various rate for more than60cycles, the capacity could recover to520mAh/g for the current of100mA/g.2. Porous NiO hollow microspheres have been synthesized via a simple hydrothermal synthesis using L-cysteine as a structure-directing agent followed by calcination. The as-synthesized NiO microspheres are hollow with diameters of2-3μm. The shells of the microspheres are built from nanoparticles with diameters of30-50nm, and the interior cavities are around0.75μm in diameter. A plausible mechanism has been proposed to explain the formation of the porous NiO hollow spheres. When evaluated as anode material for lithium ion battery, the porous NiO hollow microspheres show outstanding electrochemical performances, including high reversible capacity of847.2mAh/g after50cycles at100mA/g, high rate capability with a discharge capacity of470mAh/g at a current density of800mA/g, and good cycling stability.3. The NiO/graphene nanocomposite was prepared by solution-based method under reflux conditions and subsequent annealing. As anode for Li ion batteries, the composite has2169.6mAh/g and1467.2mAh/g specific capacities for the first discharge and charge respectively, higher than the theoretical capacity of NiO. It shows improved cycling performance with a discharge capacity of704.8mAh/g after50cycles at a current of200mA/g, and has ameliorative rate capacity with402.6mAh/g at a current of1600mA/g. This electrochemical performance is better than that of porous NiO nanosheets. |
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