Seung Mi Oh

Composition‐Tailored 2 D Mn1−xRuxO2 Nanosheets and Their Reassembled Nanocomposites: Improvement of Electrode Performance upon Ru Substitution

Composition-tailored Mn1-x Rux O2 2u2009D nanosheets and their reassembled nanocomposites with mesoporous stacking structure are synthesized by a soft-chemical exfoliation reaction and the subsequent reassembling of the exfoliated nanosheets with Li(+) cations, respectively. The tailoring of the chemical compositions of the exfoliated Mn1-x Rux O2 2u2009D nanosheets and their lithiated nanocomposites can be achieved by adopting the Ru-substituted layered manganese oxides as host materials for exfoliation reaction. Upon the exfoliation-reassembling process, the substituted ruthenium ions remain stabilized in the layered Mn1-x Rux O2 lattice with mixed Ru(3+) /Ru(4+) oxidation state. The reassembled Li-Mn1-x Rux O2 nanocomposites show promising pseudocapacitance performance with large specific capacitances of approximately 330u2005Fu2009g(-1) for the second cycle and approximately 360u2005Fu2009g(-1) for the 500th cycle and excellent cyclability, which are superior to those of the unsubstituted Li-MnO2 homologue and many other MnO2 -based materials. Electrochemical impedance spectroscopy analysis provides strong evidence for the enhancement of the electrical conductivity of 2u2009D nanostructured manganese oxide upon Ru substitution, which is mainly responsible for the excellent electrode performance of Li-Mn1-x Rux O2 nanocomposites. The results underscore the powerful role of the composition-controllable metal oxide 2u2009D nanosheets as building blocks for exploring efficient electrode materials.

A Direct Hybridization between Isocharged Nanosheets of Layered Metal Oxide and Graphene through a Surface‐Modification Assembly Process

An efficient and universal method to directly hybridize isocharged nanosheets of layered metal oxide and reduced graphene oxide (rGO) is developed on the basis of the surface modification and an electrostatically driven assembly process. On the basis of this synthetic method, the CoO2 -rGO nanocomposite can be synthesized with exfoliated CoO2 and rGO nanosheets, and transformed into CoO-CoO2 -rGO nanocomposites with excellent electrode performance for lithium-ion batteries. Also, this surface-modification assembly route is successfully applied for the synthesis of another mesoporous TiO2 -rGO nanocomposite. This result provides clear evidence for the usefulness of the present method as a universal way of hybridizing isocharged anionic nanosheets of inorganic solids and graphene.

Recent Applications of 2D Inorganic Nanosheets for Emerging Energy Storage System

Among many types of nanostructured inorganic materials, highly anisotropic 2D nanosheets provide unique advantages in designing and synthesizing efficient electrode and electrocatalyst materials for novel energy storage technologies. 2D inorganic nanosheets boast lots of unique characteristics such as high surface area, short ion diffusion path, tailorable compositions, and tunable electronic structures. These merits of 2D inorganic nanosheets render them promising candidate materials as electrodes for diverse secondary batteries and supercapacitors, and electrocatalysts. A wide spectrum of examples is presented for inorganic nanosheet-based electrodes and electrocatalysts. Future perspectives in research about 2D nanosheet-based functional materials are discussed to provide insight for the development of next-generation energy storage systems using 2D nanostructured materials.

Exfoliated clay nanosheets as an efficient additive for improving the electrode functionality of graphene-based nanocomposites

An effective and economical method to enhance the electrode activity of metal oxide–graphene nanocomposites has been developed using exfoliated clay nanosheets as an additive. The close similarities in 2D sheet-like morphologies and negative surface charges between exfoliated clay and graphene nanosheets make possible the homogeneous mixing of these two colloidal nanosheets. The crystal growth of Mn3O4 nanocrystals on the surface of negatively-charged clay/graphene nanosheets in the presence of ammonia yields Mn3O4–N-doped graphene–clay nanocomposites. The incorporation of rigid clay nanosheets is effective in decreasing the strong π–π interaction and severe aggregation of graphene nanosheets and in optimizing the porosity of the resulting nanocomposites. The clay-incorporated Mn3O4–N-doped graphene–clay nanocomposites show better electrode performance for lithium ion batteries than the clay-free homologue, underscoring the beneficial effect of clay addition on the electrochemical activity of Mn3O4–N-doped graphene nanocomposites. This is attributable to the improvement of charge transport properties and the optimization of pore structure upon the addition of clay nanosheets. The addition of clay nanosheets is also effective in improving the electrode performance of graphene-based nanocomposites for sodium ion batteries, confirming the universal usefulness of exfoliated clay nanosheets as an additive for exploring efficient electrode materials.

A 2D Metal Oxide Nanosheet as an Efficient Additive for Improving Na-Ion Electrode Activity of Graphene-Based Nanocomposites

An efficient way to improve the Na-ion electrode activity of graphene-based nanocomposite is developed by employing exfoliated metal oxide nanosheet as an additive. The titanate-nanosheet-incorporated Na-SnS2 -reduced graphene oxide (rG-O) nanocomposites can be synthesized by the electrostatically derived restacking of the colloidal mixture of SnS2 , rG-O, and titanate nanosheets with the Na+ cation. The incorporation of titanate into the Na-SnS2 -rG-O nanocomposites is effective in improving the nanoscale mixing of component nanosheets and the porosity of the composite structure. The resulting nanocomposites deliver superior discharge capacities and rate properties to the titanate-free nanocomposite. The universal applicability is further confirmed by MoS2 -rG-O nanocomposites upon the addition of titanate. This study highlights that the exfoliated metal oxide nanosheet can be used as an efficient additive for graphene-based nanocomposites to explore Na-ion electrode materials.

Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way To Improve the Na Ion Electrode Activity of Layered Titanate

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.