Synthesis of Metal Oxide/Carbon Nanofibers via Biostructure Confinement as High-Capacity Anode Materials.

For applications in energy storage and conversion, many metal oxide (MO)/C composite fibers have been synthesized using cellulose as the template. However, MO particles in carbon fibers usually experience anomalous growth to a size of >200 nm, which is detrimental to the overall performance of the composite. In this paper, we report the successful development of a generic approach to synthesize a fiber composite with highly dispersed MO nanoparticles (10-80 nm) via simple swelling, nitrogen doping, and carbonization of the cellulose microfibril. The growth of the MO nanoparticles is confined by the structure of the microfibrils. Density functional theory calculation further reveals that the doped N atoms supply ample nucleation sites for size confinement of the nanoparticles. The encapsulation structure of small MO nanoparticles in the conductive carbon matrix improves their electrochemical performance. For example, the formed SnOx/carbon nanocomposite exhibits high specific capacities of 1011.0 mA h g-1 at 0.5 A g-1 and 581.8 mA h g-1 at 5 A g-1. Moreover, the fiber-like nanocomposite can be combined with carbon nanotubes to form a flexible binder-free electrode with a capacity of ∼10 mA h cm-2, far beyond the commercial level. The process developed in this study offers an alternative approach to sophisticated electrospinning for the synthesis of other fiber-like MO/carbon nanocomposites for versatile applications.

Synthesis of Porous Si/C Composite Nanosheets from Vermiculite with a Hierarchical Structure as a High-Performance Anode for Lithium-Ion Battery.

Silicon nanosheets are fascinating anode materials for lithium-ion batteries because of their high specific capacities, structural stability, and fast kinetics in alloying/dealloying with Li. The nanosheets can be synthesized through chemical vapor deposition (CVD), topochemical reaction, and templating method. After coating with a carbon nanolayer, they exhibit enhanced electrochemical performance. However, it is challenging to synthesize ultrathin carbon-coated silicon nanosheets. In this work, porous silicon/carbon (pSi/C) composite nanosheets are synthesized by reducing the carbon-coated expanded vermiculite with metallic Al in the molten salts. The as-prepared pSi/C nanosheets retain the layered nanostructure of vermiculite, with a thickness of less than 50 nm. The carbon nanolayer serves as the diffusion barrier and mechanical support for the growth of mesoporous silicon nanosheets. The anode of pSi/C nanosheets achieves remarkable electrochemical performance, exhibiting a reversible capacity of 1837 mA h g-1 at 4 A g-1 and retaining 71.5% of the initial capacity after 500 cycles. The process can be extended to the synthesis of the pSi/C composite nanotube by using other carbon-coated silicate templates such as halloysite.

3D Interconnected V6O13 Nanosheets Grown on Carbonized Textile via a Seed-Assisted Hydrothermal Process as High-Performance Flexible Cathodes for Lithium-Ion Batteries.

Three-dimensional (3D) free-standing nanostructured materials have been proven to be one of the most promising electrodes for energy storage due to their enhanced electrochemical performance. And they are also widely studied for the wearable energy storage systems. In this work, interconnected V6O13 nanosheets were grown on the flexible carbonized textile (c-textile) via a seed-assisted hydrothermal method to form a 3D free-standing electrode for lithium-ion batteries (LIBs). The electrode exhibited a specific capacity of 170 mA h g-1 at a specific current of 300 mA g-1. With carbon nanotube (CNT) coating, its specific capacities further increased 12-40% at the various current rates. It could retain a reversible capacity of 130 mA h g-1, 74% of the initial capacity after 300 cycles at the specific current of 300 mA g-1. It outperformed most of the mixed-valence vanadium oxides. The improved electrochemical performance was ascribed to the synergistic effect of the 3D nanostructure of V6O13 for feasible Li+ diffusion and transport and highly conductive hierarchical conductive network formed by CNT and carbon fiber in c-textile.

Formation of Si Hollow Structures as Promising Anode Materials through Reduction of Silica in AlCl3-NaCl Molten Salt.

Hollow nanostructures are attractive for energy storage and conversion, drug delivery, and catalysis applications. Although these hollow nanostructures of compounds can be generated through the processes involving the well-established Kirkendall effect or ion exchange method, a similar process for the synthesis of the pure-substance one ( e. g., Si) remains elusive. Inspired by the above two methods, we introduce a continuous ultrathin carbon layer on the silica nano/microstructures (Stöber spheres, diatom frustules, sphere in sphere) as the stable reaction interface. With the layer as the diffusion mediator of the reactants, silica structures are successfully reduced into their porous silicon hollow counterparts with metal Al powder in AlCl3-NaCl molten salt. The structures are composed of silicon nanocrystallites with sizes of 15-25 nm. The formation mechanism can be explained as an etching-reduction/nucleation-growth process. When used as the anode material, the silicon hollow structure from diatom frustules delivers specific capacities of 2179, 1988, 1798, 1505, 1240, and 974 mA h g-1 at 0.5, 1, 2, 4, 6, and 8 A g-1, respectively. After being prelithiated, it retains 80% of the initial capacity after 1100 cycles at 8 A g-1. This work provides a general way to synthesize versatile silicon hollow structures for high-performance lithium ion batteries due to the existence of ample silica reactants and can be extended to the synthesis of hollow structures of other materials.