Yoon Hwa

Characterizations and electrochemical behaviors of disproportionated SiO and its composite for rechargeable Li-ion batteries

Commercially available solid SiO is composed of amorphous Si with silicon suboxides of various valence states. Solid SiO is thermodynamically unstable at all temperatures, which would disproportionate to Si and SiO2. Disproportionation of SiO at several temperatures was characterized with various analytical techniques, such as XRD, XPS, NMR and HRTEM methods, and it was found that the sample heat treated at 1000 °C contained well-developed Si nanocrystals uniformly dispersed within an amorphous SiOx matrix. Using this sample, a nano-Si/SiOx/graphite composite was obtained by a straightforward high-energy mechanical milling technique. The nano-Si/SiOx/graphite composite was tested as an anode material for Li secondary batteries, and showed better electrochemical behaviors than those of pure SiO.

SnO2@Co3O4 hollow nano-spheres for a Li-ion battery anode with extraordinary performance

AbstractSnO2@Co3O4 hollow nano-spheres have been prepared using the template-based sol-gel coating technique and their electrochemical performance as an anode for lithium-ion battery (LIB) was investigated. The size of synthesized hollow spheres was about 50 nm with the shell thickness of 7–8 nm. The fabricated SnO2@Co3O4 hollow nano-sphere electrode exhibited an extraordinary reversible capacity (962 mAh·g−1 after 100 cycles at 100 mA·g−1), good cyclability, and high rate capability, which was attributed to the Co-enhanced reversibility of the Li2O reduction reaction during cycling.

Stibnite (Sb2S3) and its amorphous composite as dual electrodes for rechargeable lithium batteries

Orthorhombic Sb2S3 (stibnite) and its amorphous Sb2S3/C composite were synthesized by solid-state routes, such as heat treatment and high energy mechanical milling (HEMM). X-ray diffraction, X-ray photoelectron spectroscopy and high resolution transmission electron microscopy confirmed that the amorphous Sb2S3/C composite was composed of amorphous Sb2S3 phases distributed uniformly within an amorphous carbon matrix. The electrochemical reaction mechanism of amorphous Sb2S3/C with Li was examined by extended X-ray absorption fine structure analyses. The fabricated amorphous Sb2S3/C composite electrode showed excellent electrochemical properties, such as a high energy density (1st charge: 757 mAh g−1), long cyclability (ca. 600 mAh g−1 over 100 cycles), and good initial Coulombic efficiency (ca. 85%). When tested within various voltage windows, the amorphous Sb2S3/C composite electrode was found to be suitable for use as a dual electrodes, such as the anode in Li-ion batteries or the cathode in Li/S batteries.

Nanostructured Zn-based composite anodes for rechargeable Li-ion batteries

The mechanism of the electrochemical reaction of Zn with Li was investigated by ex situ X-ray diffraction (XRD) analysis combined with a differential capacity plot of the Zn electrode at a low current of 10 mA g−1. The pure Zn electrode showed a high reactivity with Li, with first discharge and charge capacities of 574 and 351 mA h g−1, respectively. In addition, Zn–C and Zn–Al2O3–C composites prepared by simple high-energy mechanical milling were evaluated for use as anode materials in rechargeable Li-ion batteries. The Zn–C nanocomposite was composed of nanosized Zn in an amorphous C matrix, while the Zn–Al2O3–C nanocomposite (obtained by the mechanochemical reduction of ZnO and Al) was composed of nanocrystalline Zn and amorphous Al2O3 in the amorphous C matrix. Electrochemical tests showed that the Zn–Al2O3–C nanocomposite electrode exhibited a high volumetric capacity of more than 1800 mA h cm−3 over 100 cycles.

Nanosize Si anode embedded in super-elastic nitinol (Ni–Ti) shape memory alloy matrix for Li rechargeable batteries

A nanosize Si embedded in super-elastic Nitinol alloy matrix composite was synthesized in large-scale using arc melting followed by a rapid quenching method. Both X-ray diffraction and high resolution transmission electron microscope with energy dispersive spectroscopy analyses confirmed that approximately 50 nm Si crystallites were surrounded by the Ni–Ti matrix. Ex situsynchrotron XRD was performed to elucidate the phase transition of active materials during lithiation and delithiation. The local structural changes of the Ni–Ti inactive matrix during cycling were investigated by ex situX-ray absorption spectroscopy analyses. This anode material showed an excellent electrochemical stability since the elastic behavior of the inactive Nitinol matrix absorbed the stress generated due to the volume expansion during lithiation of the nanosized Si embedded.

Reaction mechanism and enhancement of cyclability of SiO anodes by surface etching with NaOH for Li-ion batteries

SiO has been considered as one of the most promising anodes, the reaction mechanism of SiO with Li is still not well understood. High energy mechanical milling (HEMM) followed by etching of the SiO surface with NaOH solution was carried out in this study to enhance cycle performance. X-ray photoelectron spectroscopy (XPS) analyses showed the different compositions of the Si valence state before and after sputtering, and the atomic ratio of O/Si at the surface differed from that of the inside bulk SiO. Using high resolution transmission electron microscopy (HRTEM), the reaction mechanism of a SiO electrode was investigated. During the first discharge, Li2Si2O5 and Li6Si2O7 phases were identified as well as Li4SiO4 and Li rich amorphous binary LixSi phases. The surface etched SiO showed significantly better cyclability with the reversible capacity of 1260 mA h g−1 over 50 cycles.

Reversible storage of Li-ion in nano-Si/SnO2 core–shell nanostructured electrode

A core–shell nanostructured composite material consisting of nano-Si as the core and SnO2 as the shell is synthesized by a sol–gel method. The reaction mechanism between Li and a nano-Si/SnO2 core–shell nanostructured electrode is investigated, the partial reversible reaction between Li and SnO2 during the first cycle is identified, and the reactivity of the Si core is investigated by ex situ analyses. The nano-Si/SnO2 core–shell nanostructured electrode shows a reversible capacity of ca. 1000 mA h g−1 and good cycle retention close to 80% of the first charge capacity over 50 cycles.

Effect of oxide layer thickness to nano–Si anode for Li-ion batteries

Oxide thickness controlled amorphous SiO2/nano-Si core–shell structure anode for Li ion batteries was easily prepared by etching with NaOH solution using commercial nano-Si with a native oxide. Based on X-ray diffraction patterns and analysis of high resolution transmission microscope images, the electrode which consists of amorphous SiO2/amorphous Si/core crystalline Si after the first cycle showed a self-limiting reaction behavior. Due to this unique dual core–shell structure for a short diffusion path, a high reversible capacity of about 1800 mAh g−1 over 50 cycles and excellent rate capability could be achieved.