Kwang Chul Roh

In situ synthesis of chemically bonded NaTi2(PO4)3/rGO 2D nanocomposite for high-rate sodium-ion batteries

A phase-pure NaTi2(PO4)3/reduced graphene oxide (rGO) nanocomposite was prepared using a microwave-assisted one-pot method and subsequent heat treatment. The well-crystallized NaTi2(PO4)3 nanoparticles (30–40 nm) were uniformly precipitated on rGO templates through Ti–O–C bonds. The chemical interactions between the NaTi2(PO4)3 nanoparticles and rGO could immobilize the NaTi2(PO4)3 nanoparticles on the rGO sheets, which might be responsible for the excellent electrochemical performance of the nanocomposite. The NaTi2(PO4)3/rGO nanocomposite exhibited a specific capacity of 128.6 mA·h·g–1 approaching the theoretical value at a 0.1 C-rate with an excellent rate capability (72.9% capacity retention at 50 C-rate) and cycling performance (only 4.5% capacity loss after 1,000 cycles at a high rate of 10 C). These properties were maintained even when the electrodes were prepared without the use of an additional conducting agent. The excellent sodium storage properties of the NaTi2(PO4)3/rGO nanocomposite could be attributed to the nano-sized NaTi2(PO4)3 particles, which significantly reduced the transport lengths for Na+ ions, and an intimate contact between the NaTi2(PO4)3 particles and rGO due to chemical bonding.

Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications

Graphene nanomeshes (GNMs) with nanoscale periodic or quasi-periodic nanoholes have attracted considerable interest because of unique features such as their open energy band gap, enlarged specific surface area, and high optical transmittance. These features are useful for applications in semiconducting devices, photocatalysis, sensors, and energy-related systems. Here, we report on the facile and scalable preparation of multifunctional micron-scale GNMs with high-density of nanoperforations by catalytic carbon gasification. The catalytic carbon gasification process induces selective decomposition on the graphene adjacent to the metal catalyst, thus forming nanoperforations. The pore size, pore density distribution, and neck size of the GNMs can be controlled by adjusting the size and fraction of the metal oxide on graphene. The fabricated GNM electrodes exhibit superior electrochemical properties for supercapacitor (ultracapacitor) applications, including exceptionally high capacitance (253 F g−1 at 1 A g−1) and high rate capability (212 F g−1 at 100 A g−1) with excellent cycle stability (91% of the initial capacitance after 50u2006000 charge/discharge cycles). Further, the edge-enriched structure of GNMs plays an important role in achieving edge-selected and high-level nitrogen doping.

Silica-assisted bottom-up synthesis of graphene-like high surface area carbon for highly efficient ultracapacitor and Li-ion hybrid capacitor applications

We report a facile bottom-up approach for the synthesis of pure and macro-sized (>500 nm) graphene-like carbon by precisely employing sp2 carbon rich 1,2,4,5-benzene tetracarboxylic acid (BTCA) as a precursor. We also addressed the features, such as high specific surface area (SSA) and sp2 hybridized carbon content, of the BTCA-derived carbon (BTCADC) over conventional top-down processed reduced graphene oxide (RGO). For instance, a two fold enhancement in SSA (960 m2 g−1) and Cu2006:u2006O atomic ratio (∼19) was noted for BTCADC when compared to RGO (SSA: 402 m2 g−1 and Cu2006:u2006O ratio ∼ 10). The SSA of BTCADC was further extended to 2673 m2 g−1via a chemical activation process (A-BTCADC) along with a high pore volume (2.15 cm3 g−1). Furthermore, we attempted to explain the unsolved issue of carbon layer stacking (π–π stacking) in RGO by precisely adopting a bottom-up approach. From an application point of view, we explored the possibility of using such carbonaceous materials as promising electrodes for both symmetric and Li-ion hybrid supercapacitor configurations in an organic medium. The A-BTCADC based symmetric cell in a 1 M tetraethylammonium tetrafluoroborate (TEA·BF4) in acetonitrile (ACN) electrolyte displayed a specific capacitance (Csp) of 225 F g−1 (at 0.5 A g−1) with a stable cycling profile of up to 10u2006000 cycles (at 10 A g−1) between 0 and 3 V. This bottom-up approach opens new avenues to extend graphene-based science and technology to the next level.

Defect-free solvothermally assisted synthesis of microspherical mesoporous LiFePO4/C

Recent studies have shown that the power capability of LiFePO4 is dramatically enhanced by reducing the size of the LiFePO4 particles to nanometer dimensions. Unfortunately, the resulting intrinsically low tap density of the nano-LiFePO4 cathode significantly reduces the volumetric energy density, which is a major hurdle for the successful commercialization of this material. Here, we report the facile synthesis of carbon-coated LiFePO4 (LiFePO4/C) with a mesoporous microspherical morphology via a solvothermal process, using ethanol as the sole solvent and in the absence of chelating agents. LiFePO4/C was highly crystalline and exhibited less than 1% anti-site defects, which is important for fast lithium conduction in LiFePO4. LiFePO4/C showed an excellent rate performance (86 mAh g−1 at a 20 C rate), a high retention ratio of 100% (140 mAh g−1 at a 1 C rate), and a high tap density (1.2 g cm−3). The material is thus suitable for use as a cathode in lithium-ion batteries and for high-power energy storage devices.

Spine-like Nanostructured Carbon Interconnected by Graphene for High-performance Supercapacitors

Recent studies on supercapacitors have focused on the development of hierarchical nanostructured carbons by combining two-dimensional graphene and other conductive sp2 carbons, which differ in dimensionality, to improve their electrochemical performance. Herein, we report a strategy for synthesizing a hierarchical graphene-based carbon material, which we shall refer to as spine-like nanostructured carbon, from a one-dimensional graphitic carbon nanofiber by controlling the local graphene/graphitic structure via an expanding process and a co-solvent exfoliation method. Spine-like nanostructured carbon has a unique hierarchical structure of partially exfoliated graphitic blocks interconnected by thin graphene sheets in the same manner as in the case of ligaments. Owing to the exposed graphene layers and interconnected sp2 carbon structure, this hierarchical nanostructured carbon possesses a large, electrochemically accessible surface area with high electrical conductivity and exhibits high electrochemical performance.

Lithium‐Ion Transport through a Tailored Disordered Phase on the LiNi0.5Mn1.5O4 Surface for High‐Power Cathode Materials

The phase control of spinel LiNi0.5 Mn1.5 O4 was achieved through surface treatment that led to an enhancement of its electrochemical properties. Li(+) diffusion inside spinel LiNi0.5 Mn1.5 O4 could be promoted by modifying the surface structure of LiNi0.5 Mn1.5 O4 through phosphidation into a disordered phase (Fd3m) that allows facile Li(+) transport. Phosphidated LiNi0.5 Mn1.5 O4 showed a significantly enhanced electrochemical performance, even at high rates exceeding 10u2005C, demonstrating that the improved kinetics (related to the amount of Mn(3+) ) can render LiNi0.5 Mn1.5 O4 competitive as a high-power cathode material for electric vehicles and hybrid electric vehicles.

A novel co-precipitation method for one-pot fabrication of a Co-Ni multiphase composite electrode and its application in high energy-density pseudocapacitors.

A multiphase composite film composed of nickel(II) hydroxide and cobalt(II) hydroxide was synthesized by a novel co-precipitation method. The film exhibited high capacitance and stable cyclic retention because of its 3D network nanostructure and high conductivity due to CoOOH evolved during cycling.

In situ fabrication of lithium titanium oxide by microwave-assisted alkalization for high-rate lithium-ion batteries

A phase-pure Li4Ti5O12/reduced graphene oxide nanocomposite was prepared using a simple one-pot synthesis of the Li–Ti–O precursor and subsequent heat treatment. The prepared nanocomposite delivers a reversible capacity of 168 mA h g−1 at 1 C-rate and a remarkable rate capability with 59% capacity retention at 50 C-rate.

Morphology control of three-dimensional carbon nanotube macrostructures fabricated using ice-templating method

Abstract3-Dimensional (3D) carbon nanotube (CNT) macrostructures with controlled morphologies were prepared by the ice-templating method. In order to control the assembling of the CNTs into 3D macrostructures, we systematically investigated the parameters critical to the control of the morphology of the 3D CNT macrostructures formed using the ice-templating method. It was found that process parameters such as the initial characteristics of the CNT suspension and its freezing conditions significantly affected the morphologies of the resulting CNT macrostructures. By adjusting the initial characteristics of the suspension of CNTs and its freezing conditions, we could fabricate not only regular 3D CNT macrostructures that consisted of aligned lamellae but also those that had a cellular structure. This is the first instance well-aligned, cellular CNT macrostructures have been prepared using the ice-templating method. Our approach can be used to develop the ice-templating method as a technique for fabricating 3D pore- and structure-controlled CNT macrostructures.

Synthesis of mesoporous spherical TiO2 and its application in negative electrode of hybrid supercapacitor

To fabricate a hybrid supercapacitor comprising activated carbon and TiO2 electrodes, anatase TiO2 was investigated as a candidate negative electrode material. It was synthesized by a combination of the sol-gel and microwave methods using dodecylamine as a structure directing agent. The synthesized TiO2 had uniform size (∼400 nm), a spherical morphology, a mesoporous structure (specific surface area = ∼100 m2g−1, pore size distribution = ∼10 nm), and anatase phase without any impurities. To balance the power and energy densities of positive (activated carbon) and negative electrodes (TiO2), each electrode was confirmed to have the specific capacity of about 190 mAh g−1 and 50 mAh g−1 (at 1 C-rate = 250 mA g−1) through half cell tests. While the specific capacitance (23 F cm−3) of the hybrid supercapacitor achieved about 28% increase compared to an existing supercapacitor (electric double-layer capacitor = 18 F cm−3) at a current density of 0.5 mA cm−2, it had a low retention ratio of 91% during 100 cycles due to the poor electronic conductivity of TiO2. However, it was confirmed that anatase TiO2 is suitable for use as a negative material in hybrid supercapacitors.