Dong-Won Park

Functionalized Graphene-Based Cathode for Highly Reversible Lithium–Sulfur Batteries

In this article, we highlight the salient issues in the development of lithium-sulfur battery (LSB) cathodes, present different points of view in solving them, and argue, why in the future, functionalized graphene or graphene oxide might be the ultimate solution towards LSB commercialization. As shown by previous studies and also in our recent work, functionalized graphene and graphene oxide enhance the reversibility of the charge-discharge process by trapping polysulfides in the oxygen functional groups on the graphene surface, thus minimizing polysulfide dissolution. This will be helpful for the rational design of new cathode structures based on graphene for LSBs with minimal capacity fading, low extra cost, and without the unnecessary weight increase caused by metal/metal oxide additives.

Enhanced reversible capacity of Li-S battery cathode based on graphene oxide

Lithium sulfur battery (LSB) offers several advantages such as very high energy density, low-cost, and environmental-friendliness. However, it suffers from serious degradation of its reversible capacity because of the dissolution of reaction intermediates, lithium polysulfides, into the electrolyte. To solve this limitation, there are many studies using graphene-based materials due to their excellent mechanical strength and high conductivity. Compared with graphene, graphene oxide (GO) contains various oxygen functional groups, which enhance the reaction with lithium polysulfides. Here, we investigated the positive effect of using GO mixed with carbon black on the performance of cathode in LSB. We have observed a smaller drop of capacity in GO mixed sulfur cathode. We further demonstrate that the mechanistic origin of reversibility improvement, as confirmed through CV and Raman spectra, can be explained by the stabilization of sulfur in lithium polysulfide intermediates by oxygen functional groups of GO to prevent dissolution. Our findings suggest that the use of graphene oxide-based cathode is a promising route to significantly improve the reversibility of current LSB.

Controlled Electrochemical Etching of Nanoporous Si Anodes and Its Discharge Behavior in Alkaline Si–Air Batteries

We report the fabrication of nanoporous silicon (nPSi) electrodes via electrochemical etching to form a porous Si layer with controllable thickness and pore size. Varying the etching time and ethanolic HF concentration results in different surface morphologies, with various degrees of electrolyte access depending on the pore characteristics. Optimizing the etching condition leads to well-developed nPSi electrodes, which have thick porous layers and smaller pore diameter and exhibit improved discharge behavior as anodes in alkaline Si-air cells in contrast to flat Si anode. Although electrochemical etching is effective in improving the interfacial characteristics of Si in terms of high surface area, we observed that mild anodization occurs and produces an oxide overlayer. We then show that this oxide layer in nPSi anodes can be effectively removed to produce an nPSi anode with good discharge behavior in an actual alkaline Si-air cell. In the future, the combination of high surface area nPSi anodes with nonaqueous electrolytes (e.g., room-temperature ionic liquid electrolyte) to minimize the strong passivation behavior and self-discharge in Si could lead to Si-air cells with a stable voltage profile and high anode utilization.

Quasi-Photonic Crystal Effect of TiCl3/Electrolyte Matrix in Unipolar Dye–Absorber Devices

Effectiveness of TiCl3 pre- and post-treatments on dye-sensitized solar cells (DSCs) and interfacial charge-transfer properties were investigated. It was confirmed that a yield of current collection was strongly dependent on the position of the TiCl3/electrolyte matrix in the DSC configuration. The interfacial charge-transfer properties were studied using thermionic emission-diffusion process and electrochemical impedance spectroscopy analysis. The TiCl3/electrolyte matrix was considered to be a three-dimensional quasi-photonic crystal with a photonic band gap, which reinforces electric field and facilitates current collection from the TiCl3/electrolyte matrix to the FTO by accelerating electron motion, whereas the potential barrier blocks current collection from the TiO2 bulk region to the FTO and decreases current.