Sanghun Cho

A Self-Reducible and Alcohol-Soluble Copper-Based Metal–Organic Decomposition Ink for Printed Electronics

We report a novel method for the synthesis of a self-reducible (thermally reducible without a reducing atmosphere) and alcohol-soluble copper-based metal-organic decomposition (MOD) ink for printed electronics. Alcohol-solvent-based conductive inks are necessary for commercial printing processes such as reverse offset printing. We selected copper(II) formate as a precursor and alkanolamine (2-amino-2-methyl-1-propanol) as a ligand to make an alcohol-solvent-based conductive ink and to assist in the reduction reaction of copper(II) formate. In addition, a co-complexing agent (octylamine) and a sintering helper (hexanoic acid) were introduced to improve the metallic copper film. The specific resistivity of copper-based MOD ink (Cuf-AMP-OH ink) after heat treatment at 350 °C is 9.46 μΩ·cm, which is 5.5 times higher than the specific resistivity of bulk copper. A simple stamping transfer was conducted to demonstrate the potential of our ink for commercial printing processes.

Facile Synthesis of Oxidation-Resistant Copper Nanowires toward Solution-Processable, Flexible, Foldable, and Free-Standing Electrodes

Oxidation-resistant copper nanowires (Cu NWs) are synthesized by a polyol reduction method. These Cu NWs show excellent oxidation resistance, good dispersibility, and have a low sintering temperature. A Cu NW-based flexible, foldable, and free-standing electrode is fabricated by filtration and a sintering process. The electrode also exhibits high electrical conductivity even bending, folding, and free-standing.

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 μm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω ◻(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).

All solid state flexible supercapacitors operating at 4 V with a cross-linked polymer–ionic liquid electrolyte

4 V-operated all solid symmetrical supercapacitors that employ mixtures of various weight compositions with cross-linked poly-4-vinylphenol (c-P4VPh) and 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) electrolytes have been demonstrated and characterized. The values at 1:3, 3.5, 4 and 4.5 (c-P4VPh:EMITFSI) offer free-standing membranes with high ionic conductivity. In the case of 1:3.5, the best specific capacitance (172.44 F g−1 in a single-electrode) and energy density (72.23 W h kg−1) were obtained at symmetrical cells based on porous carbon electrodes. Every prepared SC was reliable over 1000 cycles in the range of 0–4 V. They also have excellent flexibility and maintain capacitance after completing the bending test a thousand times.

Self-reducible copper ion complex ink for air sinterable conductive electrodes

Copper (Cu) based conductive inks have been widely studied with the objective of achieving highly conductive and low-cost electrodes for various electrical devices. However, the unstable oxidation properties of Cu inks make them difficult to be applied for a wide range of practical applications. The oxidation properties induce high resistivity in the electrode fabrication, and storage problem of ink. Herein, we introduce a novel self-reducible Cu ion complex ink (Cu-ink), composed by formate, alkanolamine groups and poly alcohols, for the air sinterable, low-cost, environmentally friendly fabrication of Cu conductive electrodes. The air sinterable properties of this novel Cu-ink are induced by the self-reducing activity of the Cu-ink ligand decomposition and the reduction-assistance properties of the polyol solvents. In particular, among various polyol solvents, glycerol was found to be the most suitable reduction assistant-material because of its relatively abundant hydroxyl groups, good evaporation properties, and environmentally friendly solvents. Through investigation of the Cu-ink sintering temperature and glycerol contents, we obtained the Cu electrode films with a low resistivity of 17 μΩ cm at 350 °C under air sintering conditions. Furthermore, various practical characteristics such as excellent storage stability (of up to 4 weeks), enhanced adhesion properties, and flexible retention characteristics for up to 2000 bending times (R/R0 < 1.2, bending radius 20 mm) were also demonstrated for Cu electrode films.

Enhanced electrochemical capabilities of lithium ion batteries by structurally ideal AAO separator

In this study, a novel inorganic separator, porous anodic aluminum oxide (AAO), is introduced for a rechargeable lithium ion battery system. The highly ordered AAO gives rise to an ideal structure for battery separators with appropriate porosity (67.4 %), extremely low tortuosity, and thermal durability. The prepared AAO separator has average pore sizes of 75 nm and thickness of 54 μm, which leads to enhanced ionic conductivity (2.196 mS cm−1), discharging capacity at high current rates (20.13 mA h g−1 at 10 C), and capacity retention (82.9%). Moreover, a computer simulation (COMSOL) model shows that the ideal AAO separator structure induces stable lithium ion battery operation in wide ranges of current rate, due to effective suppression of Li dendrite formation. The AAO separator has a strong potential in massive energy storage systems and electric vehicles.

Curved copper nanowires-based robust flexible transparent electrodes via all-solution approach

Curved Cu nanowire (CCN)-based high-performance flexible transparent conductive electrodes (FTCEs) were fabricated via a fully solution-processed approach, involving synthesis, coating, patterning, welding, and transfer. Each step involved an innovative technique for completing the all-solution processes. The high-quality and well-dispersed CCNs were synthesized using a multi-polyol method through the synergistic effect of specific polyol reduction. To precisely control the optoelectrical properties of the FTCEs, the CCNs were uniformly coated on a polyimide (PI) substrate via a simple meniscus-dragging deposition method by tuning several coating parameters. We also employed a polyurethane (PU)-stamped patterning method to effectively produce 20 μm patterns on CCN thin films. The CCN thin films exhibited high electrical performance, which is attributed to the deeply percolated CCN network formed via a solvent-dipped welding method. Finally, the CCN thin films on the PI substrate were partially embedded and transferred to the PU matrix to reduce their surface roughness. Through consecutive processes involving the proposed methods, a highly percolated CCN thin film on the PU matrix exhibited high optoelectrical performance (Rs = 53.48 Ω/□ at T = 85.71%), excellent mechanical properties (R/R0 < 1.10 after the 10th repetition of tape peeling or 1,000 bending cycles), and a low root-mean-square surface roughness (Rrms = 14.36 nm).

Synthesis of Cu3Sn alloy nanocrystals through sequential reduction induced by gradual increase of the reaction temperature.

Cu3Sn alloy nanocrystals are synthesized by sequential reduction of Cu and Sn precursors through a gradual increase of the reaction temperature. By transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), UV/Vis spectroscopy, and X-ray diffraction (XRD) analyses, the alloy formation mechanism of Cu3Sn nanocrystals has been studied. The incremental increase of the reaction temperature sequentially induces the reduction of Sn, the diffusion of Sn into the preformed Cu nanocrystals, resulting in the intermediate phase of Cu-Sn alloy nanocrystals, and then the formation of Cu3Sn alloy nanocrystals. We anticipate that the synthesis of Cu3Sn alloy nanocrystals encourages studies toward the synthesis of various alloy nanomaterials.

Pressure-assisted electrode fabrication using simply synthesized Cu3Sn alloy nanoparticles

Alloy nanoparticles have been widely investigated due to their unique properties and various applications. In particular, electrodes based on alloy nanomaterials can be good alternatives for advanced electrode fabrication due to their inherent, unique properties. Herein, we suggest a simple one-pot synthesis of Cu3Sn nanoparticles with high oxidation-resistivity, straightforward processability, and good electron conductivity. In addition, we introduce fabrication of pressure-assisted electrodes at room temperature with the synthesized Cu3Sn nanoparticles, which have good potential for various functional devices on a plastic substrate.

Copper nanowire/multi-walled carbon nanotube composites as all-nanowire flexible electrode for fast-charging/discharging lithium-ion battery

A novel lightweight three-dimensional (3D) composite anode for a fast-charging/discharging Li-ion battery (LIB) was fabricated entirely using one-dimensional (1D) nanomaterials, i.e., Cu nanowires (CuNWs) and multi-walled C nanotubes (MWCNTs). Because of the excellent electrical conductivity, high-aspect ratio structures, and large surface areas of these nanomaterials, the CuNW-MWCNT composite (CNMC) with 3D structure provides significant advantages regarding the transport pathways for both electrons and ions. As an advanced binder-free anode, a CuNW-MWCNT composite film with a controllable thickness (∼600 μm) exhibited a considerably low sheet resistance, and internal cell resistance. Furthermore, the random CuNW network with 3D structure acting as a rigid framework not only prevented MWCNT shrinkage and expansion due to aggregation and swelling but also minimized the effect of the volume change during the charge/discharge process. Both a half cell and a full cell of LIBs with the CNMC anode exhibited high specific capacities and Coulombic efficiencies, even at a high current. More importantly, we for the first time overcame the limitation of MWCNTs as anode materials for fast-charging/discharging LIBs (both half cells and full cells) by employing CuNWs, and the resulting anode can be applied to flexible LIBs. This innovative anode structure can lead to the development of ultrafast chargeable LIBs for electric vehicles.