Kyung-Sik Hong

Multilayered 2D Sn nanodendrites were synthesized by a one-step electrodeposition process from an aqueous solution containing coumarin (C₉H₆O₂). Under a high cathodic current density that is necessary for dendritic growth, hydrogen bubbles evolved with Sn reduction. The hydrogen bubbles limited the diffusion path of the Sn ions to space between hydrogen bubbles and concealed the splitting sites for additional branches, which suppressed the development of the branches. The addition of coumarin greatly reduced the departure diameter of the hydrogen bubbles and thus made the growth of the Sn dendrites with a high density of nanosized branches possible. Furthermore, coumarin suppressed Sn crystallite growth along the [001] direction, promoting two-dimensional growth of the 2D Sn nanodendrites along the set of [220] directions that are normal to the [001] direction. These 2D Sn nanodendrites grew layer-by-layer on a substrate, forming a unique multilayered structure. As an anode material for Na-ion batteries, the multilayered 2D Sn nanodendrites exhibited a high specific capacity of 783.88 mA h g⁻¹ and excellent cycle performance with 96.6% capacity retention over 300 cycles. Moreover, the multilayered 2D Sn nanodendrites showed a superior rate capability of 412.84 mA g⁻¹ at 5C. The outstanding performance is attributed to a high density of free space inside the multilayered 2D Sn nanodendrites and the nanosized Sn branches that offer short diffusion paths for Na ions.

The present study investigates the degradation mechanism of a methanesulfonic acid (MSA) based electroplating bath used for the electrodeposition of Sn–Ag alloy solder bumps, and its effects on the microstructure of the solder and on the collapse failure of flip-chip solder bumps. To examine the degradation behavior of the electroplating bath, a degraded electrolyte is prepared by accelerated aging treatment. In the presence of dissolved oxygen and Ag⁺ ions in the electrolyte, the chemical oxidation of Sn²⁺ ions to Sn⁴⁺ ions and the precipitation of SnO₂ nanoparticles with a diameter below 100 nm are promoted by the reduction of Ag⁺. Under cathodic bias, colloidal SnO₂ particles are adsorbed on the surface of the Sn–Ag solder bumps via electrophoresis, and incorporated into the layer by the electrodeposition layer of Sn–Ag. The presence of oxide layer mainly composed of SnO₂ on the surface of the bumps significantly reduces the friction coefficient of the solder surface by hardening the electrodeposits and deteriorates the solderability of the solder bumps, which leads to collapse failure during solder reflow.

Sb/Sb₂O₃ composites are synthesized by a one-step electrodeposition process from an aqueous electrolytic bath containing a potassium antimony tartrate complex. The synthesis process involves the electrodeposition of Sb simultaneously with the chemical deposition of Sb₂O₃, which allows for the direct deposition of morula-like Sb/Sb₂O₃ particles on the current collector without using a binder. Structural characterization confirms that the Sb/Sb₂O₃ composites are composed of approximately 90 mol % metallic Sb and 10 mol % crystalline Sb₂O₃. The composite exhibits a high reversible capacity (670 mAh g^–1) that is higher than the theoretical capacity of Sb (660 mAh g^–1). The high reversible capacity results from the conversion reaction between Na₂O and Sb₂O₃ that occurs additionally to the alloying/dealloying reaction of Sb with Na. Moreover, the Sb/Sb₂O₃ composite shows excellent cycle performance with 91.8% capacity retention over 100 cycles, and a superior rate capability of 212 mAh g^–1 at a high current density of 3300 mA g^–1. The outstanding cycle performance is attributed to an amorphous Na₂O phase generated by the conversion reaction, which inhibits agglomeration of Sb particles and acts as an effective buffer against volume change of Sb during cycling.

The improved durability of the current collector for high power electrochemical devices is achieved by manipulating the Au coating layer using the electrochemical deposition method.

Three‐dimensional porous Sb/Sb₂O₃ anode materials are successfully fabricated using a simple electrodeposition method with a polypyrrole nanowire network. The Sb/Sb₂O₃–PPy electrode exhibits excellent cycle performance and outstanding rate capabilities; the charge capacity is sustained at 512.01 mAh g⁻¹ over 100 cycles, and 56.7% of the charge capacity at a current density of 66 mA g⁻¹ is retained at 3300 mA g⁻¹. The improved electrochemical performance of the Sb/Sb₂O₃–PPy electrode is attributed not only to the use of a highly porous polypyrrole nanowire network as a substrate but also to the buffer effects of the Sb₂O₃ matrix on the volume expansion of Sb. Ex situ scanning electron microscopy observation confirms that the Sb/Sb₂O₃–PPy electrode sustains a strong bond between the nanodeposits and polypyrrole nanowires even after 100 cycles, which maintains good electrical contact of Sb/Sb₂O₃ with the current collector without loss of the active materials.

Sn nanofibers with a high aspect ratio are successfully synthesized using a simple electrodeposition process from an aqueous solution without the use of templates. The synthetic approach involves the rapid electrochemical deposition of Sn accompanied by the strong adsorption of Triton X-100, which can function as a growth modifier for the Sn crystallites. Triton X-100 is adsorbed on the {200} crystallographic planes of Sn in an elongated configuration and suppressed the preferential growth of Sn along the [100] direction. Consequently, the Sn electrodeposits are forced to grow anisotropically in a direction normal to the (112) or (1̅12) plane, forming one-dimensional nanofibers. As electrode materials for the Na-ion batteries, the Sn nanofibers exhibit a high reversible capacity and an excellent cycle performance; the charge capacity is maintained at 776.26 mAh g^–1 after 100 cycles, which corresponds to a retention of 95.09% of the initial charge capacity. The superior electrochemical performance of the Sn nanofibers is mainly attributed to the high mechanical stability of the nanofibers, which originate from highly anisotropic expansion during sodiation and the pore volumes existing between the nanofibers.

Structurally stabilized Na₃V₂(PO₄)₃/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na₃V₂(PO₄)₃ structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na₃V₂(PO₄)₃ cathode materials for Na-ion batteries.