Dohyeon Yoon

Supercritical Carbon Dioxide-Assisted Process for Well-Dispersed Silicon/Graphene Composite as a Li ion Battery Anode.

The silicon (Si)/graphene composite has been touted as one of the most promising anode materials for lithium ion batteries. However, the optimal fabrication method for this composite remains a challenge. Here, we developed a novel method using supercritical carbon dioxide (scCO2) to intercalate Si nanoparticles into graphene nanosheets. Silicon was modified with a thin layer of polyaniline, which assisted the dispersion of graphene sheets by introducing π-π interaction. Using scCO2, well-dispersed Si/graphene composite was successfully obtained in a short time under mild temperature. The composite showed high cycle performance (1,789 mAh/g after 250 cycles) and rate capability (1,690 mAh/g at a current density of 4,000 mA/g). This study provides a new approach for cost-effective and scalable preparation of a Si/graphene composite using scCO2 for a highly stable lithium battery anode material.

A simple, one-pot synthesis of molybdenum oxide-reduced graphene oxide composites in supercritical methanol and their electrochemical performance

A simple and green supercritical methanol (scMeOH) route is developed to tightly anchor molybdenum oxide (MoO2) nanoparticles on reduced graphene oxide (RGO). In scMeOH, graphene oxide is reduced, and MoO2 nanoparticles with sizes of 10–20 nm are simultaneously deposited on the basal plane of RGO in a short time without using any reducing agents or additives. When tested as an anode in lithium ion batteries, the MoO2–RGO composites show enhanced electrochemical performance compared to bare MoO2. The composite with a MoO2 loading of 37.0 wt% delivers a high reversible discharge capacity of 793 mA h g−1 at 50 mA g−1 and an excellent rate performance of 205 mA h g−1 at 2.5 A g−1. After 100 cycles of high rate testing of up to 50 A g−1, the MoO2–RGO composite recovers most of its initial capacity. The improved electrochemical performance of MoO2–RGO can be attributed to the tight anchoring of nanosized MoO2 on RGO and the mesoporous structure of the composite. Consequently, the transport length of Li diffusion into the MoO2 phase is shortened, charge transfer kinetics at the electrode–electrolyte interface is facilitated, and the volume expansion associated with the conversion reaction can be accommodated.

Carbon with expanded and well-developed graphene planes derived directly from condensed lignin as a high-performance anode for sodium-ion batteries

In this study, we demonstrate that lignin, which constitutes 30-40 wt % of the terrestrial lignocellulosic biomass and is produced from second generation biofuel plants as a cheap byproduct, is an excellent precursor material for sodium-ion battery (NIB) anodes. Because it is rich in aromatic monomers that are highly cross-linked by ether and condensed bonds, the lignin material carbonized at 1300 °C (C-1300) in this study has small graphitic domains with well-developed graphene layers, a large interlayer spacing (0.403 nm), and a high micropore surface area (207.5 m2 g-1). When tested as an anode in an NIB, C-1300 exhibited an initial Coulombic efficiency of 68% and a high reversible capacity of 297 mA h g-1 at 50 mA g-1 after 50 cycles. The high capacity of 199 mA h g-1 at less than 0.1 V with a flat voltage profile and an extremely low charge-discharge voltage hysteresis (<0.03 V) make C-1300 a promising energy-dense electrode material. In addition, C-1300 exhibited an excellent high-rate performance of 116 mA h g-1 at 2.5 A g-1 and showed stable cycling retention (0.2% capacity decay per cycle after 500 cycles). By comparing the properties of the lignin-derived carbon with oak sawdust-derived and sugar-derived carbons and a low-temperature carbonized sample (900 °C), the reasons for the excellent performance of C-1300 were determined to result from facilitated Na+-ion transport to the graphitic layer and the microporous regions that penetrate through the less defective and enlarged interlayer spacings.