Cheolsoo Jung

A fast and efficient pre-doping approach to high energy density lithium-ion hybrid capacitors

We demonstrate that the internal short (IS) approach is a fast and efficient process for lithium pre-doping in lithium-ion capacitors. Direct contact between the graphite electrode and lithium metal leads to very fast but controllable lithium pre-doping into graphite due to the large contacting area in the excess electrolyte medium, facilitating not only fast lithium intercalation into graphite but also fast dissipation of reaction heat generated during this process. LIC cells pre-doped through the IS method exhibit remarkably higher coulombic efficiency and longer cycle life than those of the cells prepared using conventional pre-doping methods such as electrochemical (EC) and external short circuit (ESC) methods. These results indicate that the IS pre-doping approach can significantly improve the anode manufacturing speed and reduce the cost of high energy density lithium-ion capacitors.

Graphene Electrodes for Artificial Muscles

Reduced graphene oxides (RGOs) thin films were fabricated in order to apply for the compliant electrodes of artificial muscles. The RGO was chemically prepared by chemical oxidation of natural graphite via Hummers method and following reduction using hydrazine. The RGO electrodes were coated on both sides of polydimethylsiloxane (PDMS) by using spray coater. The resulting PDMS dielectric elastomers with RGO electrodes had larger electromechanical strain response than those with conventional carbon paste electrodes. It might be caused by higher conductivity and higher flexibility of RGO than those of conventional electrodes.

Modification of electrolyte transport within the cathode for high-rate cycle performance of Li-ion battery

During high-rate cycling of Li-ion batteries (LIBs) at elevated temperatures, the detachment of the cathode materials from their Al substrate is a major cause of the deterioration in the performance of LIBs. This detachment is suppressed by the addition of an electrolyte additive, poly(ethylene glycol) methyl ether methacrylate, which can act as a buffer zone to prevent the abrupt mass transport of electrolyte within the cathode and as a swing to transport Li+ ions dissociating from the active materials of the cathode. Owing to the dual effects of this type of monomer, an acrylate monomer with one side ether chain, the cathode materials are maintained without detachment from the Al substrate, even under severe cycling conditions. This idea can be applied to LIBs for a series of electric vehicles, which require superior high-rate performance at elevated temperatures.

Orientational Changes of Side Chain Nonlinear Chromophore in Polyimide Langmuir-Blodgett Films

The orientational changes in optical nonlinear active side chains of polyamic acid alkyl amine salt monolayers before and after the imidization process were studied by optical surface second-harmonic generation (SHG). The polymeric Langmuir-Blodgett (LB) monolayer composed of the triphenylamino-(p-nitrophenyl)-ethylene group as a side chain of polyamic acid was prepared by the precursor method. The analysis of the experimental results showed that the symmetry of the side chain unit in the monolayer remains C∞v after imidization process. Moreover, the side chain molecular orientational distribution functions (ODFs) of the polymeric monolayer before and after imidization were obtained using the modified maximum entropy method. From the obtained ODFs, it was found that the polar angle of the side chain increases by imidization and this causes the decrement of the nonlinear optical components. However the half-width (Δθ) of the major molecular tilt angle decreases by imidization, suggesting that the distribution of the chromophore is more uniformly packed through the imidization process.