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Submicron and micron-sized carbon spheres with high electrical conductivity, micro–mesoporous structure, and empty space are considered ideal sulfur host materials for Li–S batteries. In this study, a new and efficient strategy for the synthesis of yolk-shell-structured carbon microspheres with multishells is introduced. Pitch-derived carbon microspheres with yolk-shell structure, high pore volume, and high electrical conductivity are synthesized by applying yolk-shell-structured Fe2O3 microspheres as a sacrificial template. Pitch-infiltrated Fe2O3 microspheres transform into yolk-shell-structured carbon (YS_C) microspheres with a specific conductivity of 0.9 S cm−1 after post-treatment and etching with an HCl solution. The discharge capacity of the sulfur-loaded YS_C microspheres for the 200th cycle at a current density of 0.5 C is 686 mA h g−1, and their reversible capacity after 800 cycles at a high current density of 2 C is 412 mA h g−1. In addition, the sulfur-loaded YS_C microspheres show excellent cycling performance, despite their very high sulfur loading of 70 wt%. The excellent cycling and rate performances of the sulfur-loaded YS_C microspheres are attributed to the synergistic effect of the high electrical conductivity of YS_C microspheres, loading of amorphous and ultrafine sulfur, and empty shell layers.

SnO2-based composite materials have been studied as efficient anode materials for lithium-ion batteries. In this study, pitch-derived carbon coated SnO2–CoO yolk–shell microspheres were synthesized by a spray drying process. Pitch is a widely used source material for electrically conductive carbon. Pitch-infiltrated SnO2–Co3O4 were transformed into SnO2–CoO–C yolk–shell microspheres by a carbothermal reduction. SnO2–CoO–C yolk–shell microspheres with a carbon content of 15 wt% exhibited superior cycling and rate performances compared with those of the bare SnO2–Co3O4 microspheres with the same morphologies. The discharge capacities of SnO2–Co3O4 and SnO2–CoO–C at the 100th cycle were 565 and 812 mA h g−1, while their capacity retentions calculated from the second cycle were 51 and 97%, respectively. Furthermore, SnO2–CoO–C yolk–shell microspheres exhibited high and stable reversible capacities even at an extremely high current density of 30 A g−1. The discharge capacity of SnO2–CoO–C yolk–shell microspheres at the 1000th cycle at a current density of 3.0 A g−1 was 775 mA h g−1. The synergetic effect of the pitch-derived carbon with a high electrical conductivity, catalytic effect of the metallic Co, crystal growth minimization of metallic Co and Sn by reciprocal action, and yolk–shell structure with empty shells provided the SnO2–CoO–C yolk–shell microspheres with excellent lithium-ion storage performances.

Silicon is considered an important industrial element, primarily owing to its use in semiconductors, which are key components of various electronic devices; thus, the development of low-cost and efficient purification silicon processes has become a research focus. In this study, we developed a new process using ammonium fluoride (NH4F) to remove residual silica (SiO2) and generate nanopores on raw silicon (Si) materials without the use of highly toxic reagents and expensive procedures. We conducted low-temperature heat treatment of raw Si with NH4F under an inert atmosphere and found that the purity of raw Si was improved to a level similar to that of raw Si treated with hydrofluoric acid solution. Moreover, through this process, the surface area of raw Si was increased by surface tearing and the formation of nanopores. We also proposed a reaction mechanism for removing residual SiO2 from raw Si through X-ray diffraction analysis and demonstrated the safety of the process by analyzing the by-products generated during the heat treatment. When the modified Si material was applied to Li-ion battery anodes, they showed improved capacity, initial coulombic efficiency, and cycle performance as compared with those using raw Si material. We expect that this new NH4F-based method will not only be used to modify various Si materials for their efficient application, but will also be used to obtain nanostructured materials that require the removal of SiO2 during synthesis.