WonKeun Kim

Synergistic effects of various morphologies and Al doping of spinel LiMn2O4 nanostructures on the electrochemical performance of lithium-rechargeable batteries

Nanostructured electrodes have recently received great attention as components in lithium rechargeable batteries, especially because of the high power produced by the fast kinetic properties of these unique structures. Here, we report the successful synthesis of various nanostructured morphologies of spinel lithium manganese oxide electrodes (nanorod, nanothorn sphere, and sphere) from a similarly shaped manganese dioxide precursor that was controlled with different aluminium contents by the hydrothermal method. Among these structures, nanothorn sphere structured LiAl0.02Mn1.98O4 produces the highest discharge capacity of 129.8 mA h g−1, excellent rate capability (94.6 mA h g−1 at 20 C, 72% of 0.2 C-rate discharge capacity) and stable cyclic retention for 50 cycles. The excellent kinetic properties of the nanothorn sphere structure are not only due to the nanothorn sphere electrode having high surface area but also because the critical amount of Al in the nanothorn sphere electrode was located at the Mn site (16d) instead of the Li site (8a).

Effects of compositional and structural change on the corrosion behaviour of nitrogen implanted Zircaloy-4

Abstract The influence of nitrogen implantation on the corrosion behaviour of Zircaloy-4 was examined by potentiodynamic polarization tests in a chloride and an acid solution, and the results were discussed with structural and compositional variations of implanted layer that was determined by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The resistance to localized corrosion of Zircaloy-4 was very sensitive to the ion dose and the substrate temperature during the implantation. At substrate temperatures above 200°C, the pitting potential of the nitrogen implanted alloy in deaerated 4 M NaCl at 80°C increased gradually with the ion dose from 350 mVSCE for the unimplanted sample to 990 mVSCE for the alloy implanted with an ion dose of 6×1017 ions cm−2, and then the alloy was immune to pitting when implanted with ion doses greater than about 1×1018 ions cm−2. In contrast to this, the alloy implanted at 100°C exhibited an inferior corrosion resistance to the unimplanted sample irrespective of the ion dose. A drastic increase in the resistance to pitting corrosion in chlorides as well as a significant reduction in the passive current density in acid solution of the implanted alloy, were found to be associated with the formation of ZrN layer with a stoichiometric ratio of N to Zr. The low resistance to localized and general corrosion of the alloy implanted at 100°C was attributed to the increase in structural defects produced by ion bombardment, and to low atomic mobility in the implanted layer.

Tailoring Crystal Structure and Morphology of LiFePO4/C Cathode Materials Synthesized by Heterogeneous Growth on Nanostructured LiFePO4 Seed Crystals

Porous and coarse (5-10 μm) LiFePO₄/C composites with excellent electrochemical performance were synthesized by a growth technology using nanostructured (100-200 nm) LiFePO₄ as seed crystals for the 2nd crystallization process. The porous and coarse LiFePO₄/C presented a high initial discharge capacity (∼155 mA h g⁻¹ at 0.1 C), superior rate-capability (∼100 mA h g⁻¹ at 5 C, ∼65 % of the discharge capacity at 0.1 C), and excellent cycling performance (∼131 mA h g⁻¹, ∼98 % of its initial discharge capacity after 100 cycles at 1 C). The improvement in the rate-capability of the LiFePO₄/C was attributed to the high reaction area resulted from the pore tunnels formed inside LiFePO₄ particles and short Li-ion diffusion length. The improved cycling performance of the LiFePO₄/C resulted from the enhanced structural stability against Li-deficient LiFePO₄ phase formation after cycling by the expansion of the 1D Li-ion diffusion channel in the LiFePO₄ crystal structure.