One-Step Production of Anisotropically Etched Graphene Using Supercritical Water

We developed a one-step method for production of anisotropically etched graphene using supercritical fluid (SCF). Anisotropic etching of a graphite substrate and dispersed graphite powder with Ag nanoparticles was conducted in supercritical water (SCW). Because of the exfoliation effect of SCF, graphene was isolated from the graphite simultaneously with the anisotropic etching. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy revealed the production of multilayer graphene exfoliated from the anisotropically etched graphite surface

Analysis of Degradation Mechanisms in Quinone-Based Electrodes for Aqueous Electrolyte System via <i>In Situ</i> XRD Measurements

Organic materials are promising electroactive components of energy storage devices such as lithium-ion batteries and electrochemical capacitors. Among them, low-molecular-weight organics have attracted attention as higher-energy-density, environmentally friendly, and inexpensive electrode materials, but their poor cycle performance is the main drawback. Using <i>in situ</i> XRD measurement in aqueous electrolyte system, here we investigated the capacity fading mechanism of an organic electrode based on low-molecular-weight quinones. Although the capacity fading of such organic electrodes is generally attributed to their elution into the electrolyte, our structural analysis reveals that the capacity fading is also associated with the expansion of an electrochemically inactive region, which persists in the electrode but does not take part in the reversible redox reactions. Moreover, the detailed analysis of the XRD patterns suggests that the capacity fading of the electrode is accompanied by the crystal growth of organic component, which occurs through dissolution–reprecipitation processes taking place during charge–discharge cycling. The association between capacity fading and the increased size of these crystalline domains suggests that the elongated electrical/ionic conduction paths in the growing organic crystals (leading to the expansion of the electrochemically inactive region of the electrode) can be a possible capacity fading mechanism in organic electrodes

Ultrathin SnS₂ Nanoparticles on Graphene Nanosheets: Synthesis, Characterization, and Li-Ion Storage Applications

Ultrathin SnS₂ nanoparticle decorated graphene nanosheet (GNS) electrode materials with delaminated structure were prepared using stepwise chemical modification of graphene oxide (GO) nanosheets at very dilute conditions, followed by a hydrothermal treatment. The chemical modification of the graphene nanosheet surface with Sn ions enables the precipitation of ultrathin nanoparticles. The TEM analysis reveals the SnS₂ nanoparticles are homogeneously distributed on the loosely packed graphene surface in such a way that the GNS restacking was hindered. X-ray photoelectron spectroscopic analysis reveals the bonding characteristics of the SnS₂ on the GNS. The obtained nanocomposite exhibits a reversible capacity of 1002 mAh/g, which is significantly higher than its calculated theoretical capacity (584 mAh/g). Furthermore, its cycling performance is enhanced and after 50 cycles, and the charge capacity still remained 577 mAh/g, which is very close to its theoretical capacity. Due to the synergic effect, the Li-ion storage capacity observed for nanocomposites is much higher than its theoretical capacity. The ultrathin size (2 nm) and dimensional confinement of tin sulfide nanoparticles by the surrounding GNS limit the volume expansion upon lithium insertion, and the nanoporous structures serve as buffered spaces during charge/discharge and result in superior cyclic performances by facilitating the electrolyte to contact the entire nanocomposite materials and reduce lithium diffusion length in the nanocomposite

Relocation of Cobalt Ions in Electrochemically Delithiated LiCoPO₄ Cathode Materials

Relocation of Cobalt Ions in Electrochemically Delithiated LiCoPO₄ Cathode Material

Electronic States of Quinones for Organic Energy Devices: The Effect of Molecular Structure on Electrochemical Characteristics

The molecular design of organic energy-storage devices relies on correlations between the electrochemical properties of organic materials and their molecular structures. Here we report a systematic study of the fundamental electronic states of the quinone family of redox-active materials. Poly­(ethylene oxide) coatings, as elution inhibitors, facilitated the evaluation of the electrochemical properties of single quinone molecules. Moreover, we confirmed experimentally how LUMO energies and their corresponding redox potentials depend on molecular structure, including the number of aromatic rings, the positions of functional groups, and coordination structures; this was achieved by elemental and chemical-state-selective X-ray absorption spectroscopy, and DFT calculations. We introduce an energy diagram depicting a segmentalized reduction process; this diagram considers the intermediate states during redox reactions to discuss processes that dominate changes in electrochemical properties as molecular structures are altered. Our results and analysis strategy are widely applicable to the material design of future organic molecular-based devices

Exfoliated MoS₂ and MoSe₂ Nanosheets by a Supercritical Fluid Process for a Hybrid Mg–Li-Ion Battery

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS₂ and MoSe₂ nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS₂ and MoSe₂ nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg–Li-ion battery as a proof of functionality. The MoS₂ and MoSe₂ nanosheets exhibited the specific capacities of 81 and 55 mA h g^–1, respectively, at a current rate of 20 mA g^–1

Unravelling the Surface Structure of MgMn₂O₄ Cathode Materials for Rechargeable Magnesium-Ion Battery

The spinel MgMn₂O₄, a cathode material with theoretical capacity of 272 mA h g^–1, holds promise for future application in high volumetric magnesium-ion batteries. Atomic-resolution imaging of the structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn₂O₄ for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO₂ was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries

Unravelling the Surface Structure of MgMn₂O₄ Cathode Materials for Rechargeable Magnesium-Ion Battery

The spinel MgMn₂O₄, a cathode material with theoretical capacity of 272 mA h g^–1, holds promise for future application in high volumetric magnesium-ion batteries. Atomic-resolution imaging of the structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn₂O₄ for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO₂ was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries

Disulfide-Bridged (Mo₃S₁₁) Cluster Polymer: Molecular Dynamics and Application as Electrode Material for a Rechargeable Magnesium Battery

Exploring novel electrode materials is critical for the development of a next-generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo₃S₁₁) clusters, has great potential as a rechargeable magnesium battery cathode. This material provided good reversible capacity, attributed to its unique structure with high flexibility and capability of deformation upon Mg insertion. Free-terminal disulfide moiety may act as the active site for reversible insertion and extraction of magnesium

Disulfide-Bridged (Mo₃S₁₁) Cluster Polymer: Molecular Dynamics and Application as Electrode Material for a Rechargeable Magnesium Battery

Exploring novel electrode materials is critical for the development of a next-generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo₃S₁₁) clusters, has great potential as a rechargeable magnesium battery cathode. This material provided good reversible capacity, attributed to its unique structure with high flexibility and capability of deformation upon Mg insertion. Free-terminal disulfide moiety may act as the active site for reversible insertion and extraction of magnesium