Enhanced reversible lithium storage in a nanosize silicon/graphene composite

Li ion battery materials with core-shell nanostructures

Nanomaterials have some disadvantages in application as Li ion battery materials, such as low density, poor electronic conductivity and high risk of surface side reactions. In recent years, materials with core-shell nanostructures, which was initially a common concept in semiconductors, have been introduced to the field of Li ion batteries in order to overcome the disadvantages of nanomaterials, and increase their general performances in Li ion batteries. Many efforts have been made to exploit core-shell Li ion battery materials, including cathode materials, such as lithium transition metal oxides with varied core and shell compositions, and lithium transition metal phosphates with carbon shells; and anode materials, such as metals, alloys, Si and transition metal oxides with carbon shells. More recently, graphene has also been proposed as a shell material. All these core-shell nanostructured materials presented enhanced electrochemical capacity and cyclic stability. In this review, we summarize the preparation, electrochemical performances, and structural stability of core-shell nanostructured materials for lithium ion batteries, and we also discuss the problems and prospects of this kind of materials.

Graphene-encapsulated hollow Fe₃O₄ nanoparticle aggregates as a high-performance anode material for lithium ion batteries

Graphene-encapsulated ordered aggregates of Fe(3)O(4) nanoparticles with nearly spherical geometry and hollow interior were synthesized by a simple self-assembly process. The open interior structure adapts well to the volume change in repetitive Li(+) insertion and extraction reactions; and the encapsulating graphene connects the Fe(3)O(4) nanoparticles electrically. The structure and morphology of the graphene-Fe(3)O(4) composite were confirmed by X-ray diffraction, scanning electron microscopy, and high-resolution transmission microscopy. The electrochemical performance of the composite for reversible Li(+) storage was evaluated by cyclic voltammetry and constant current charging and discharging. The results showed a high and nearly unvarying specific capacity for 50 cycles. Furthermore, even after 90 cycles of charge and discharge at different current densities, about 92% of the initial capacity at 100 mA g(-1) was still recoverable, indicating excellent cycle stability. The graphene-Fe(3)O(4) composite is therefore a capable Li(+) host with high capacity that can be cycled at high rates with good cycle life. The unique combination of graphene encapsulation and a hollow porous structure definitely contributed to this versatile electrochemical performance.

Graphene-wrapped TiO2 hollow structures with enhanced lithium storage capabilities

In this work, we report a rational design of graphene sheets-wrapped anatase TiO(2) hollow particles. This unique hybrid structure demonstrates significantly enhanced lithium storage capabilities compared to the pure TiO(2) counterpart, which clearly uncovers the merit of structural design and rational integration with graphene sheets.

Nanographene-constructed carbon nanofibers grown on graphene sheets by chemical vapor deposition: high-performance anode materials for lithium ion batteries

We report on the fabrication of 3D carbonaceous material composed of 1D carbon nanofibers (CNF) grown on 2D graphene sheets (GNS) via a CVD approach in a fluidized bed reactor. Nanographene-constructed carbon nanofibers contain many cavities, open tips, and graphene platelets with edges exposed, providing more extra space for Li(+) storage. More interestingly, nanochannels consisting of graphene platelets arrange almost perpendicularly to the fiber axis, which is favorable for lithium ion diffusion from different orientations. In addition, 3D interconnected architectures facilitate the collection and transport of electrons during the cycling process. As a result, the CNF/GNS hybrid material shows high reversible capacity (667 mAh/g), high-rate performance, and cycling stability, which is superior to those of pure graphene, natural graphite, and carbon nanotubes. The simple CVD approach offers a new pathway for large-scale production of novel hybrid carbon materials for energy storage.

Graphene enhances Li storage capacity of porous single-crystalline silicon nanowires

We demonstrated that graphene significantly enhances the reversible capacity of porous silicon nanowires used as the anode in Li-ion batteries. We prepared our experimental nanomaterials, viz., graphene and porous single-crystalline silicon nanowires, respectively, using a liquid-phase graphite exfoliation method and an electroless HF/AgNO3 etching process. The Si porous nanowire/graphene electrode realized a charge capacity of 2470 mAh g(-1) that is much higher than the 1256 mAh g(-1) of porous Si nanowire/C-black electrode and 6.6 times the theoretical capacity of commercial graphite. This relatively high capacity could originate from the favorable charge-transportation characteristics of the combination of graphene with the porous Si 1D nanostructure.

Deformations in Si-Li anodes upon electrochemical alloying in nano-confined space

The energy density of Li-ion batteries can be increased if graphitic anodes are replaced with nanostructured Si-based materials. Design of efficient Si anodes requires a better fundamental understanding of the possible changes in Si-Li alloy morphology during cycling. Here we propose a simple elastoplastic model to predict morphological changes in Si upon electrochemical reaction with Li in a confined geometry, such as a pore of a carbon nanotube (CNT). Our experiments with CNTs having inner Si coatings of different thicknesses confirmed the theoretical predictions and demonstrated irreversible shape changes in the first cycle and fully reversible shape changes in subsequent cycles. During the first lithiation, Si was found to adapt to the restricted shape of the rigid CNT pore and plastically deform during electrochemical alloying with Li. The sequential Li insertion and extraction periodically alters the tube size between the expanded and contracted states. The produced samples of porous Si with rigid CNT outer shell showed capacity up to 2100 mAh/g, stable performance for over 250 cycles, and outstanding average Coulombic efficiency in excess of 99.9%. CNT walls were demonstrated to withstand stresses caused by the initial Si expansion and Li intercalation.

Graphene anchored with co(3)o(4) nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance

We report a facile strategy to synthesize the nanocomposite of Co(3)O(4) nanoparticles anchored on conducting graphene as an advanced anode material for high-performance lithium-ion batteries. The Co(3)O(4) nanoparticles obtained are 10-30 nm in size and homogeneously anchor on graphene sheets as spacers to keep the neighboring sheets separated. This Co(3)O(4)/graphene nanocomposite displays superior Li-battery performance with large reversible capacity, excellent cyclic performance, and good rate capability, highlighting the importance of the anchoring of nanoparticles on graphene sheets for maximum utilization of electrochemically active Co(3)O(4) nanoparticles and graphene for energy storage applications in high-performance lithium-ion batteries.