Pulverization Control by Confining Fe3O4 Nanoparticles Individually into Macropores of Hollow Carbon Spheres for High-Performance Li-Ion Batteries.

In this article, double carbon shell hollow spheres which provide macropores (mC) for ultrasmall Fe3O4 nanoparticle (10-20 nm) encapsulation individually were first prepared (Fe3O4@mC). The well-constructed Fe3O4@mC electrode materials offer the feasibility to study the volume change, aggregation, and pulverization process of the active Fe3O4 nanoparticles for Li-ion storage in a confined space. Fe3O4@mC exhibits excellent electrochemical performances and delivers a high capacity of 645 mA h g-1 at 2 A g-1 after 1000 cycles. Even at 10 A g-1 or after 1000 cycles at 2 A g-1, the porous carbon structure was well maintained and no obvious aggregation and pulverization of the Fe3O4 nanoparticles was observed, although the volume of the active Fe3O4 particles was expanded to 40-60 nm compared to that of the original particles (10-20 nm). This can be due to the in situ embedment of one Fe3O4 nanoparticle into one macropore individually. The uniform dispersion and confinement of the Fe3O4 nanoparticles in the macropores of the carbon shell could effectively accommodate severe volume variations upon cycling and prevent self-aggregation and spreading out from the carbon shell during the expansion process of the nanoscale Fe3O4 particles, leading to improved capacity retention. Our work confirms the effectiveness for pulverization control by confining Fe3O4 nanoparticles individually into macropores to improve its Li-ion storage properties, providing a novel strategy for the design of new-structured anode materials for Li-ion batteries.

Interpenetrated Networks between Graphitic Carbon Infilling and Ultrafine TiO2 Nanocrystals with Patterned Macroporous Structure for High-Performance Lithium Ion Batteries.

Interpenetrated networks between graphitic carbon infilling and ultrafine TiO2 nanocrystals with patterned macropores (100-200 nm) were successfully synthesized. Polypyrrole layer was conformably coated on the primary TiO2 nanoparticles (∼8 nm) by a photosensitive reaction and was then transformed into carbon infilling in the interparticle mesopores of the TiO2 nanoparticles. Compared to the carbon/graphene supported TiO2 nanoparticles or carbon coated TiO2 nanostructures, the carbon infilling would provide a conductive medium and buffer layer for volume expansion of the encapsulated TiO2 nanoparticles, thus enhancing conductivity and cycle stability of the C-TiO2 anode materials for lithium ion batteries (LIBs). In addition, the macropores with diameters of 100-200 nm in the C-TiO2 anode and the mesopores in carbon infilling could improve electrolyte transportation in the electrodes and shorten the lithium ion diffusion length. The C-TiO2 electrode can provide a large capacity of 192.8 mA h g-1 after 100 cycles at 200 mA g-1, which is higher than those of the pure macroporous TiO2 electrode (144.8 mA h g-1), C-TiO2 composite electrode without macroporous structure (128 mA h g-1), and most of the TiO2 based electrodes in the literature. Importantly, the C-TiO2 electrode exhibits a high rate performance and still delivers a high capacity of ∼140 mA h g-1 after 1000 cycles at 1000 mA g-1 (∼5.88 C), suggesting good lithium storage properties of the macroporous C-TiO2 composites with high capacity, cycle stability, and rate capability. This work would be instructive for designing hierarchical porous TiO2 based anodes for high-performance LIBs.

Multishelled Nickel-Cobalt Oxide Hollow Microspheres with Optimized Compositions and Shell Porosity for High-Performance Pseudocapacitors.

Nickel-cobalt oxides/hydroxides have been considered as promising electrode materials for a high-performance supercapacitor. However, their energy density and cycle stability are still very poor at high current density. Moreover, there are few reports on the fabrication of mixed transition-metal oxides with multishelled hollow structures. Here, we demonstrate a new and flexible strategy for the preparation of hollow Ni-Co-O microspheres with optimized Ni/Co ratios, controlled shell porosity, shell numbers, and shell thickness. Owing to its high effective electrode area and electron transfer number (n(3/2) A), mesoporous shells, and fast electron/ion transfer, the triple-shelled Ni-Co1.5-O electrode exhibits an ultrahigh capacitance (1884 F/g at 3A/g) and rate capability (77.7%, 3-30A/g). Moreover, the assembled sandwiched Ni-Co1.5-O//RGO@Fe3O4 asymmetric supercapacitor (ACS) retains 79.4% of its initial capacitance after 10 000 cycles and shows a high energy density of 41.5 W h kg(-1) at 505 W kg(-1). Importantly, the ACS device delivers a high energy density of 22.8 W h kg(-1) even at 7600 W kg(-1), which is superior to most of the reported asymmetric capacitors. This study has provided a facile and general approach to fabricate Ni/Co mixed transition-metal oxides for energy storage.