Wook Ahn

Sulfur Nanogranular Film-Coated Three-Dimensional Graphene Sponge-Based High Power Lithium Sulfur Battery

To meet the requirements of both high energy and power density with cycle durability of modern EVs, we prepared a novel nanosulfur granular assembled film coated on the three-dimensional graphene sponge (3D-GS) composite as a high-performance active material for rechargeable lithium sulfur batteries. Instead of conventional graphene powder, three-dimensional rGO sponge (3D-rGO) is employed for the composite synthesis, resulting in a sulfur film directly in contact with the underlying graphene layer. This significantly improves the overall electrical conductivity, strategically addressing challenges of conventional composites of low sulfur utilization and dissolution of polysulfides. Additionally, the synthesis mechanism of 3D-GS is elucidated by XPS and DFT analyses, where replacement of hydroxyl group of 3D-rGO sponge by sulfur (S8) is found to be thermodynamically favorable. As expected, 3D-GS demonstrates outstanding discharge capacity of 1080 mAh g(-1) at a 0.1C rate, and 86.2% capacity retention even after 500 cycles at a 1.0C rate.

Interaction mechanism between a functionalized protective layer and dissolved polysulfide for extended cycle life of lithium sulfur batteries

Correction for ‘Interaction mechanism between a functionalized protective layer and dissolved polysulfide for extended cycle life of lithium sulfur batteries’ by Wook Ahn et al., J. Mater. Chem. A, 2015, 3, 9461–9467.

Preparation of a Reduced Graphene Oxide Wrapped Lithium-Rich Cathode Material by Self-Assembly

A lithium-rich cathode material wrapped in sheets of reduced graphene oxide (RGO) and functionalized with polydiallyldimethylammonium chloride (PDDA) was prepared by self-assembly induced from the electrostatic interaction between PDDA-RGO and the Li-rich cathode material. At current densities of 1000 and 2000 mA g(-1), the PDDA-RGO sheet wrapped samples demonstrated increased discharge capacities, increasing from 125 to 155 mA h g(-1) and from 82 to 124 mA h g(-1), respectively. The decreased resistance implied by this result was confirmed from electrochemical impedance spectroscopy results, wherein the charge-transfer resistance of the pristine sample decreased after wrapping with the PDDA-RGO sheets. The PDDA-RGO sheets served as a protective layer sand as a conductive material, which resulted in an improvement in the retention capacity from 56 to 81% after 90 cycles.

Multigrain electrospun nickel doped lithium titanate nanofibers with high power lithium ion storage

Novel in situ nickel doped 1-D lithium titanate nanofibers (Li4Ti5−xNixO12, where x = 0, 0.05 and 0.1) have been successfully synthesized using a facile electrospinning process. Physical characterization reveals that nickel is homogeneously incorporated into the lattice of lithium titanate nanofibers (LTONFs) which significantly improves their properties yielding outstanding electrochemical performance in a lithium ion battery at high power rates and significant reduction in the voltage gap between the oxidation and reduction peaks. A capacity of 190 mA h g−1 has been obtained at 0.2C for the 10% nickel doped nanofibers (Ni-LTONF10), which is higher than the theoretical capacity of pristine lithium titanate (175 mA h g−1) and they also show superior rate capability resulting in 63 mA h g−1 obtained at 50C, which is 20 times higher than that of un-doped pristine LTONFs and lithium titanate nanoparticles (LTONPs). Finally, a hybrid supercapacitor is fabricated using Ni-LTONF10, showing superior energy density at high power density.

Highly Oriented Graphene Sponge Electrode for Ultra High Energy Density Lithium Ion Hybrid Capacitors

Highly oriented rGO sponge (HOG) can be easily synthesized as an effective anode for application in high-capacity lithium ion hybrid capacitors. X-ray diffraction and morphological analyses show that successfully exfoliated rGO sponge on average consists of 4.2 graphene sheets, maintaining its three-dimensional structure with highly oriented morphology even after the thermal reduction procedure. Lithium-ion hybrid capacitors (LIC) are fabricated in this study based on a unique cell configuration which completely eliminates the predoping process of lithium ions. The full-cell LIC consisting of AC/HOG-Li configuration has resulted in remarkably high energy densities of 231.7 and 131.9 Wh kg(-1) obtained at 57 W kg(-1) and 2.8 kW kg(-1). This excellent performance is attributed to the lithium ion diffusivity related to the intercalation reaction of AC/HOG-Li which is 3.6 times higher that of AC/CG-Li. This unique cell design and configuration of LIC presented in this study using HOG as an effective anode is an unprecedented example of performance enhancement and improved energy density of LIC through successful increase in cell operation voltage window.

Carbide-derived carbon/sulfur composite cathode for multi-layer separator assembled Li-S battery

To improve the electrochemical performance of Li-S rechargeable batteries, tunable porous carbon materials, which are known as carbide-derived carbons (CDCs), are employed as adsorbents and conductive matrices for the cathodic sulfur materials. A new assembly for Li-S cells was developed by introducing multi-layer membranes as separators. The use of the multi-layer membranes enables the minimization of the shuttle effect by expanding the distance between the separators and blocking the penetration of the polysulfide. The best discharge capacity and cycle life were obtained with ten layers of PP membrane in a sulfur-CDC@1200 composite cathode, resulting in a discharge capacity of 670 mA h g−1 and a minimal gap in the charge-discharge capacity during cell cycling.

Modified chalcogens with a tuned nano-architecture for high energy density and long life hybrid super capacitors

A novel in situ vanadium-modified NiCo2S4 wrapped with graphene sheets was synthesized using a simple solvothermal technique. The vanadium-modified sample (VNCS) demonstrated superior electrochemical performance over graphene-wrapped NiCo2S4 (GNCS) and pure NiCo2S4 (NCS) samples, with a specific capacitance of 1340 F g−1 at a current density of 2 A g−1. The VNCS sample also showed outstanding capacitance retention at 50 A g−1 (1024 F g−1), which is 430% higher than that of the NCS sample. The cycling stability of VNCS was significantly improved by more than 140% compared to the NCS sample with less than 10−3 F per cycle loss in capacitance after 10 000 cycles. We also report the fabrication of hybrid supercapacitors (HSCs) using three different materials for the faradaic electrode. The VNCS-HSCs showed significant improvements in all electrochemical performance measurements compared to NCS-HSCs. The capacitance retention at 50 A g−1 was improved by more than 260% and the long cycling stability was improved by more than 180% after 10 000 cycles. The VNCS-HSC delivered an energy density of 45.9 W h kg−1 at 0.87 kW kg−1 and maintained a superior energy density of 33.6 W h kg−1 at 9 kW kg−1 indicating the excellent potential of this material in hybrid super capacitor applications.

Elevated rate capability of sulfur wrapped with thin rGO layers for lithium–sulfur batteries

Sulfur–rGO composite is prepared by a facile solution method using carbon disulfide solvent as a cathode electrode material for lithium–sulfur batteries to investigate its electrochemical performance. The initial capacity of the sulfur–rGO composite is 1385 mA h g−1 obtained at 0.1 C-rate with an excellent average capacity reduction ratio of 0.37% per cycle (from 10th to 100th cycles).

Hierarchical Core–Shell Nickel Cobaltite Chestnut-like Structures as Bifunctional Electrocatalyst for Rechargeable Metal–Air Batteries

Nano-engineered hierarchical core-shell nickel cobaltite chestnut-like structures were successfully synthesized as a bifunctionally active electrocatalyst for rechargeable metal-air battery applications. Both the morphology and composition of the catalyst were optimized by a facile hydrothermal reaction, resulting in a 10 h reacted sample demonstrating significantly enhanced activity toward both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in 0.1 m KOH. Specifically, the catalyst demonstrated -0.28 and 0.60 V versus SCE (saturated calomel electrode) at the ORR half-wave potential and an OER current density of 10 mA cm-2 , respectively. The resulting ORR/OER potential difference of 0.90 V was the smallest compared to the catalysts synthesized using 2, 6, and 12 h of hydrothermal reaction time. The excellent bifunctional activity of the catalyst is attributed to the nanoscale porous morphology and the spinel nickel cobaltite composition, which improved the active site exposure and transport of reactants and charges during the oxygen reactions.

Self-Assembly of Spinel Nanocrystals into Mesoporous Spheres as Bifunctionally Active Oxygen Reduction and Evolution Electrocatalysts

The present work introduces spinel oxide nanocrystals self-assembled into mesoporous spheres that are bifunctionally active towards catalyzing both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The electrochemical evaluation reveals that (Ni,Co)3 O4 demonstrates a significantly positive-shifted ORR onset and half-wave potentials [-0.127 and -0.292 V vs. saturated calomel electrode (SCE), respectively], whereas Co3 O4 results in a negative-shifted OER potential (0.65 V vs. SCE) measured at 10 mA cm-2 . Based on the DFT analysis, the potential at which all oxygen intermediate reactions proceed spontaneously is the highest for (Ni,Co)3 O4 (U=0.66 eV) during ORR, whereas it is the lowest for Co3 O4 (U=2.09 eV) during OER. The high ORR activity of (Ni,Co)3 O4 is attributed to the enhanced electrical conductivity of the spinel lattice, and the high OER activity of Co3 O4 is attributed to relatively weak adsorption energy promoting rapid release of evolved oxygen.