Ji Woo Kim

Tunable Sn structures in porosity-controlled carbon nanofibers for all-solid-state lithium-ion battery anodes

Volumetric expansion of active materials during lithium (Li) insertion is a critical drawback of Li-alloy anodes and a major bottleneck for their wide adoption in rechargeable batteries. Here, we report on a novel fabrication method of a tin (Sn) fully embedded one-dimensional (1D) carbon (C) matrix which results in minimal volumetric expansion. The 1D C matrix contributes to the buffer role and electron conduction path. This optimized Sn/C structure is enabled by confining the Sn nucleation site and minimizing the outward Sn diffusion originating from stress relaxation. The difference of thermal expansion coefficient between Sn and C derives the stress. The porosity of C nanofibers is a key parameter to modulate the Sn size and dispersion. It is controlled by stabilization and gas–solid reactions between CO (g), CO2 (g), and C nanofibers. The calcination under an Ar environment, which induced the lowest surface area and total pore volume (10.46 m2 g−1 and 0.0217 cm3 g−1), creates an ideal structure of 15 nm sized uniform Sn nanoparticle embedded C nanofibers. It displays a superior anode performance in all-solid-state Li-ion batteries with a capacity of 762 mA h g−1 and coulombic efficiency greater than 99.5% over 50 cycles. Our scheme provides a fundamental impact on anode materials of Li-ion batteries.

34.2: PMMA Buffer‐Layer Effects on Electrical Performance of Pentacene OTFTs with a Cross‐linked PVA Gate Insulator on a Flexible Substrate

In this paper we proposed for the first time a technique of poly-methylmethacrylate (PMMA) buffer layer insertion to overcome unsaturated output characteristics of OTFTs with cross-linked poly-vinylalcohol (PVA) gate insulators on a PET substrate. A 3:1 diluted PMMA buffer layer insertion resulted in saturated output characteristics and small shift of a threshold voltage in successive I-V measurements for OTFTs due to the enhancement of surface hydrophobicity on the gate insulator. The electrical performances of a threshold voltage, a subthreshold slope and on-off current ratio have been noticeably improved after PMMA buffer layers insertion. To our best knowledge, the highest mobility of 0.32 cm2/Vsec has been obtained among OTFTs fabricated with polymer gate insulators by spin coating processes on a PET substrate.

A Generic Approach for Embedded Catalyst-Supported Vertically Aligned Nanowire Growth

We demonstrate a general approach for growing vertically aligned, single-crystalline nanowires of any material on arbitrary substrates by using plasma-sputtered Au/Pd thin films as a catalyst through the vapor-liquid-solid process. The high-energy sputtered Au/Pd atoms form a reactive interface with the substrate forming nanoclusters which get embedded in the substrate, thus providing mechanical stability for vertically aligned nanowire growth. We demonstrate that our approach for vertically aligned nanowire growth is generic and can be extended to various complex substrates such as conducting indium tin oxide.

The effect of energetically coated ZrOx on enhanced electrochemical performances of Li(Ni1/3Co1/3Mn1/3)O2 cathodes using modified radio frequency (RF) sputtering

To date, most coating layers for electrode materials for Li-ion batteries have been fabricated using the sol–gel method or atomic layer deposition (ALD), which involve complicated processing steps and limited candidates for coating materials. With an emphasis on solving these issues, herein, a new coating methodology based on a sputtering system was developed, and sputtered zirconium oxide was coated on Li(Ni1/3Co1/3Mn1/3)O2 (L333) cathode powders. The continuous movement of the cathode powders during the coating procedure and the high kinetic energy from the sputtering process resulted in a highly uniform coating layer with multiple structures exhibiting a concentration and valence state gradient of Zr, i.e., surface (mainly Zr4+) and doped (mainly Zr2+) layers. The ZrOx-coated L333 powders exhibited an outstanding capacity retention (96.3% at the 200th cycle) and superior rate capability compared with the uncoated version in a coin cell with 1 M LiPF6 in EC:DEC liquid electrolyte. The ZrOx-coated L333 powders also exhibited an enhanced specific capacity in a solid state battery cell with a sulfide-based inorganic solid-state electrolyte. The improved electrochemical performance of ZrOx/L333 was attributed to the synergetic effect from the surface and doped layers: physical/chemical protection of the active material surface, enhancement of Li-ion diffusion kinetics, and stabilization of the interfaces.

Microstructural characterization of dehydrogenated products of the LiBH4-YH3 composite.

The dehydrogenated microstructure of the lithium borohydride-yttrium hydride (LiBH4-YH3) composite obtained at 350°C under 0.3 MPa of hydrogen and static vacuum was investigated by transmission electron microscopy combined with a focused ion beam technique. The dehydrogenation reaction between LiBH4 and YH3 into LiH and YB4 takes place under 0.3 MPa of hydrogen, which produces YB4 nano-crystallites that are uniformly distributed in the LiH matrix. This microstructural feature seems to be beneficial for rehydrogenation of the dehydrogenation products. On the other hand, the dehydrogenation process is incomplete under static vacuum, leading to the unreacted microstructure, where YH3 and YH2 crystallites are embedded in LiBH4 matrix. High resolution imaging confirmed the presence of crystalline B resulting from the self-decomposition of LiBH4. However, Li2B12H12, which is assumed to be present in the LiBH4 matrix, was not clearly observed.