Young-Sik Hong

Synthesis and electrochemical properties of nanocrystalline Li[NixLi(1−2x)/3Mn(2−x)/3]O2 prepared by a simple combustion method

Nanocrystalline Li[NixLi(1−2x)/3Mn(2−x)/3]O2 powders were prepared by a simple combustion method and investigated using X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), particle size analysis (PSA), and galvanostatic charge/discharge cycling. According to the XRD analysis, single-phase compounds with a layered structure were obtained for powders with 0 ≤ x ≤ 0.25, while mixtures were obtained for powders with 0.30 ≤ x ≤ 0.50. Rietveld analysis revealed that single-phase Li[NixLi(1−2x)/3Mn(2−x)/3]O2 is basically a layered rock-salt structure in which a small amount of Ni occupies the 3a sites. The initial discharge capacity of a Li/Li[NixLi(1−2x)/3Mn(2−x)/3]O2 cell with x = 0.20 was about 288 mA h g−1, corresponding to about 91% of the theoretical value, when it was cycled in the voltage range of 4.8–2.0 V with a specific current of 20 mA g−1 at 30 °C. As far as we know, charge/discharge cycling on an Li/Li[Ni0.20Li0.20Mn0.60]O2 cell gives the highest discharge capacity of 288 mA h g−1 among the LiMO2-based (M = Co, Ni, and Mn) cathode materials. A very promising factor for high-rate capability applications was an excellent rate capability in continuous cycling at specific currents ranging from 20 mA g−1 to 900 mA g−1, due to the nanocrystalline particle size of 80–200 nm. The origin of the 4.5 V plateau was investigated by means of weight loss measurement and XAS for the charged/discharged electrodes. The weight loss measurement for the charged electrodes gave indirect evidence that the 4.5 V plateau did not originate from the ejection of oxygen. In XAS, the Mn oxidation state of 4+ did not change during the charge/discharge process, and surprisingly the Ni did not further oxidize beyond about 3+.

Amorphous FePO4 as 3 V cathode material for lithium secondary batteries

The structural and electrochemical properties of amorphous FePO4·xH2O (x = 2, 1, 0) and hexagonal FePO4 powders have been investigated using differential thermal analysis–thermogravimetry (DTA–TGA), X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), cyclic voltammetry (CV), and charge/discharge cycling. On heating, amorphous FePO4·2H2O was transformed into hexagonal FePO4 at 380 °C, through amorphous FePO4·H2O and FePO4 phases. Reversible lithium insertion and extraction into and from the amorphous FePO4 occurred at 2.8 and 3.2 V vs. Li/Li+, respectively, which potentials are lower than those of the olivine-structured LiFePO4. All samples show a large capacity loss at the 1st cycle, but their discharge capacities are gradually increased from 65 mA h g−1 (2nd cycle) to 75 mA h g−1 (15th cycle) at a current density of 17 mA g−1 and kept up to the 50th cycle. The discharge/charge dependence on the current densities (17, 34, and 85 mA g−1) and operating temperatures (20, 50, and 80 °C) were also investigated for amorphous FePO4. Fe K-edge X-ray absorption spectroscopy has been performed on amorphous LiyFePO4, to determine the changes in the local electronic and geometric structures during the discharge. For the Fe K-edge, the pre-edge and edge are shifted in accordance with the oxidation state of the Fe3+/Fe2+ redox couple in LiyFePO4.

Washing Effect of a LiNi0.83Co0.15Al0.02O2 Cathode in Water

The formation of LiOH and Li 2 CO 3 impurities on high Ni content LiNi 0 . 8 3 Co 0 . 1 5 Al 0 . 0 2 O 2 powders due to H 2 O and CO 2 absorption from the air can be reduced without structural degradation by washing in water. Although the as-synthesized sample had a moisture content of 570 ppm immediately after firing, this level increased rapidly to 1270 ppm in air with a relative humidity of 50%. However, its content was decreased to 210 ppm after washing twice in water, followed by heat-treatment at 700°C. It is believed that this improvement was due to the decreased level of Li 2 CO 3 and LiOH impurities on the particles. This was highlighted by the decreasing swelling of the Li-ion cell at 90°C, and the thickness of the cell containing the washed samples was decreased by 50% compared with the bare sample.

Synthesis and Electrochemical Properties of Nanocrystalline Li [ Ni0.20Li0.20Mn0.60 ] O 2

A nanocrystalline Li[Ni 0 . 2 0 Li 0 . 2 0 Mn 0 . 6 0 ]O 2 powder was synthesized by a simple combustion method and investigated using X-ray diffraction, scanning electron microscopy, and galvanostatic charge/discharge cycling. The particle size of the powder was distributed in the range of 100-200 nm. Rietveld analysis revealed that Li[Ni 0 . 2 0 Li 0 . 2 0 Mn 0 . 6 0 ]O 2 is basically a layered rock-salt structure in which a small amount of Ni occupies the 3a sites. The initial discharge capacity of the Li/Li[Ni 0 . 2 0 Li 0 . 2 0 Mn 0 . 6 0 ]O 2 cell was about 288 mAh/g when it was cycled at a voltage range of 4.8-2.0 V with a specific current of 20 mA g - 1 . A very promising factor for high-rate capability applications was a reversible capacity of 200 mAh g - 1 at the 100th cycle with a high specific current of 400 mA g - 1 . The weight loss measurement for charged Li 1 - x [Ni 0 . 2 0 Li 0 . 2 0 Mn 0 . 6 0 ]O 2 electrodes gave indirect evidence that the long plateau at 4.5 V did not originate from the ejection of oxygen.

RF-Sputtered Vanadium Oxide Films Effect of Film Thickness on Structural and Electrochemical Properties

Structural and electrochemical properties of radio frequency (rf)-sputtered vanadium oxide films with different thicknesses (0.175, 0.35, 0.8, 1.6, and 3.2 μm) have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance spectroscopy, and a galvanostatic charge/discharge system. XRI) and SEM studies show that change in the film thickness by deposition time has a significant influence on the crystalline phase and orientation of vanadium oxide films: c axis oriented V 6 O 13 phase is converted to the V 2 O 5 phase with the ab plane perpendicular to the substrate with increasing film thickness. Similar discharge profiles are observed regardless of film thickness, whereas the first discharge capacities are higher for the thin films (∼100 μ Ah/cm 2 μm for 0.175, 0.35, and 0.8 μm) than those for the thicker films (∼82 μ Ah/cm 2 μm for 1.6 and 3.2 μm) at constant current of 40 μA/cm 2 . Moreover the films with thickness of 1.6 and 3.2 μm show unstable cycle characteristics. From a long-term cycling performance for the samples with the thickness less than 0.8 μm, a large decrease in discharge capacity is observed after 1st cycle followed by almost constant value retained up to 1000th cycle. Comparison of impedance spectra for the as-prepared and the cycled 175 nm thick film reveals that there is no significant change in the charge-transfer resistance before and after cycling, indicating that capacity loss during cycling is mainly attributed to an irreversible phase transformation. However, with increasing film thickness, charge-transfer resistance at the electrolyte/electrode is found to have an influence, gradually, on the degree of capacity loss.

Crystalline Fe3PO7 as an electrode material for lithium secondary batteries

Abstract A crystalline Fe3PO7 was synthesized by the solid-state reaction method and investigated using the X-ray diffraction (XRD) method, scanning electron microscopy (SEM), cyclic voltammetry (CV) and a galvanostatic discharge–charge cycler for its application as an electrode material in lithium secondary batteries. Rietveld refinement of the powder XRD patterns yielded lattice constants of a=8.006(3) and c=6.863(3) A with the space group R3m. The lithium insertion into the Fe3PO7 electrode showed a large first-discharge capacity of about 800 mA h g−1, accompanied by a reversible charge capacity as high as 500 mA h g−1 in a voltage window of 3.5–0.5 V. During the first discharge–charge cycle, an ex situ XRD method was applied for the lithium inserted–extracted electrodes, LixFe3PO7, to investigate the structural changes. As the electrode was discharged to 0.5 V, the crystal structure of the Fe3PO7 collapsed into an amorphous phase because of the reduction of the Fe3+ to a metallic Fe. Despite the structural collapse, a crystalline phase was partially recovered in the subsequent charge.