Facile synthesis of core-shell structured Si@graphene balls as a high-performance anode for lithium-ion batteries.

Encapsulating silicon (Si) nanoparticles with graphene nanosheets in a microspherical structure is proposed to increase electrical conductivity and solve stability issues when using Si as an anode material in lithium-ion batteries (LIBs). Currently the main strategies to produce high-quality Si-graphene (Si@Gra) electrodes are (1) chemical vapor deposition (CVD) of graphene grown in situ on Si by hydrocarbon precursors and (2) encapsulating Si with a graphene oxide followed by postannealing. However, both methods require a high-temperature and are costly and time-consuming procedures, which hinders their mass scalability and practical utilization. Herein, we report a Si@Gra composite with a ball-like structure that is assembled by a facile spray drying process without a postannealing treatment. The graphene sheets are synthesized by an electrochemical exfoliation method from natural graphite. The resulting Si@Gra composite exhibits a unique core-shell structure, from which the ball-like morphology and the number of graphene layers in the Si@Gra composites are found to affect both the electric conductivity and ionic conductivity. The Si@Gra composites are found to increase the capacity of the anode and provide excellent cycling stability, which is attributed to the high electrical conductivity and mechanical flexibility of the layered graphene; additionally, a void space in the core-shelled ball structure inside the Si@Gra compensates for the Si volume expansion. As a result, the Si@few-layer graphene ball anode exhibits a high initial discharge capacity of 2882.3 mA h g-1 and a high initial coulombic efficiency of 86.9% at 0.2 A g-1. The combination of few-layer graphene sheets and the spray drying process can effectively be applied for large-scale production of core-shell structured Si@Gra composites as promising anode materials for use in high-performance LIBs.

Nanocatalyst-Assisted Fine Tailoring of Pore Structure in Holey-Graphene for Enhanced Performance in Energy Storage.

Nanoporous holey-graphene (HG) shows potential versatility in several technological fields, especially in biomedical, water filtration, and energy storage applications. Particularly, for ultrahigh electrochemical energy storage applications, HG has shown promise in addressing the issue of low gravimetric and volumetric energy densities by boosting of the ion-transport efficiency in a high-mass-loaded graphene electrode. However, there are no studies showing complete control over the entire pore architecture and density of HG and their effect on high-rate energy storage. Here, we report a unique and cost-effective method for obtaining well-controlled HG, where a copper nanocatalyst assists the predefined porosity tailoring of the HG and leads to an extraordinary high pore density that exceeds 1 × 103 µm-2. The pore architectures of the hierarchical and homogenous pores of HG were realized through a rationally designed nanocatalyst and the annealing procedure in this method. The HG electrode with a high mass loading results in improved supercapacitor performance that is at least 1 order of magnitude higher than conventional graphene flakes (reduced electrochemically exfoliated graphene (rECG)) in areal capacitance (∼100% retention of capacitance until 15 000 cycles), energy density, and power density. The diffusion coefficient of the HG electrode is 1.5-fold higher than that of rECG at a mass loading of 15 mg cm-2, indicating excellent ion-transport efficiency. The excellent ion-transport efficiency of HG is further proved by nearly 4-fold magnitude lowering of its Rion (the ionic resistance in the electrolyte-filled pores) value as compared with rECG when estimated for equivalent high-mass-loaded electrodes. Furthermore, the HG exhibits a packing density that is 2 orders of magnitude higher than rECG, revealing the utility of the maximum electrode mass and possessing higher volumetric capacitance. The perfect tailoring of HG with optimized porosity allows the achievement of high areal capacitance and excellent cycling stability due to the facile ion- and charge-transport at high-mass-loaded electrodes, which could open a new avenue for addressing the long-existing issue of practical application of graphene-based energy storage devices.

Controlled multimodal hierarchically porous electrode self-assembly of electrochemically exfoliated graphene for fully solid-state flexible supercapacitor.

Supercapacitors constructed from three-dimensional (3D) graphene electrodes with high ion-accessible surface area and durable mechanical flexibility have great potential for wearable devices. For the development of a highly efficient graphene electrode for electrical double layer capacitors (EDLCs), proper control over not only the specific surface area but also the type of pore (macro-, meso- and micro-porous networks) adapted for an appropriate type of electrolyte is crucial to ensure an ideal performance in terms of both energy density and power delivery rate. However, there is still a lack of technology to create graphene structures that combine macro-, meso- and micro-pores by a one-step and facile method. In addition, the ion/electron transport of a solid state electrolyte among such multimodal pore structures is not fully investigated. Here, we report a novel cost-effective technique of concentration dependent self-assembly of electrochemically exfoliated graphene (EC-graphene) to obtain a 3D architecture with controllable macropores (0.39-4.99 µm) and multimodal hierarchical meso- and micro-pores. The better performance of the 3D architecture is obtained due to its optimum micron-sized macropore diameter (∼5 µm) that serves as an ion buffering reservoir, followed by facile ion diffusion kinetics through the well-modulated combination of macro-, meso- and micro-pores. The binder and conductive carbon additive free supercapacitor constructed from the 3D graphene electrode exhibited a specific capacitance of 45.40 F g-1 (6 M KOH) and 23.89 F g-1 (1 M H2SO4 gel electrolyte). A capacitance retention of above 90% (up to 180° folding angle) after 50 bending-relaxing cycles is obtained, implying the superior durability of the device and the worthiness of the synthesis procedure. The method reported here may pave the way for the development of an environment friendly, large scale producible and controlled porous graphene-based architecture for the high performance next generation flexible, all-solid-state and binder-free energy storage devices.