Jinsub Lim

Enhanced High-Rate Performance of Li4Ti5O12 Nanoparticles for Rechargeable Li-Ion Batteries

Li 4 Ti 5 O 12 was successfully synthesized by solvothermal techniques using cost-effective precursors in polyol medium. The x-ray diffraction (XRD) pattern of the sample (LTO-500) was clearly indexed to the spinel shaped Li 4 Ti 5 O 12 and in order to accurately determine the lattice parameters, synchrotron powder XRD pattern was fitted by the whole-pattern profile matching method using the model space group, Fd3m. The particle size, morphology, and crystallinity of LTO-500 were identified using field-emission scanning electron microscopy and transmission electron microscopy. The electrochemical performance of the sample revealed fairly high initial discharge/charge specific capacities of 230 and 179 mAh/g, respectively, and exhibited highly improved rate performances at C-rates as high as 30 and 60 C, when compared to Li 4 Ti 5 O 12 by the solid-state reaction method. This was attributed to the achievement of small particle sizes in nanoscale dimensions, a reasonably narrow particle size distribution and, hence, shorter diffusion paths combined with larger contact area at the electrode/electrolyte interface.

Particle Size Effect of Anatase TiO2 Nanocrystals for Lithium-Ion Batteries

Nanocrystalline anatase TiO 2 was synthesized from a triethylene glycol solution of titanium isopropoxide [Ti(O-iPr) 4 ] by refluxing at 270°C for 12 h. The thermal stability and effect of particle size on the corresponding electrochemical performances were investigated by annealing the prepared sample at various temperatures; namely, 300, 400, 500, 600, 700, 800, and 900°C. The X-ray diffraction patterns of the samples clearly revealed that the maximum temperature for the formation of pure anatase phase was 700°C beyond which the presence of rutile polymorph became significant. The field emission-transmission electron microscopy images of the obtained samples showed uniform and considerably dispersed particles with fairly homogeneous distributions and average sizes of 8-50 nm. The electrochemical measurements indicated considerable charge-discharge capacities devoid of major capacity fading during extended cycles, due to their electrochemically beneficial highly crystalline traits, nanosized particles, and uniform distribution.

Fully activated Li2MnO3 nanoparticles by oxidation reaction

Fully activated Li2MnO3 nanoparticles were prepared by a chemical based oxidation reaction. All of the diffraction peaks of the prepared samples were well matched to a monoclinic phase (space group: C2/m) with no impurity peaks and refined using the General Structure Analysis System (GSAS) program. The activated Li2MnO3 sample showed homogeneously well-dispersed nanoparticles with a size of ∼10 nm. The oxidation state of Mn was confirmed by XPS. The activated Li2MnO3 nanoparticles delivered a high charge capacity of 302 mA h g−1 above 4.5 V and discharge capacity of 236 mA h g−1 during the first cycle. Interestingly, the cycle performance of the activated Li2MnO3 nanoparticles during extended cycles exhibited somewhat stable discharge capacities without any drastic capacity fading, even when cycled in the high voltage range of 2.0–4.9 V and after the phase transition to spinel. In terms of the rate performance, the activated Li2MnO3 sample exhibited significantly superior properties compared to the bulk Li2MnO3 sample, probably due to the nano-size particles with high crystallinity.

Pyro-Synthesis of Functional Nanocrystals

Despite nanomaterials with unique properties playing a vital role in scientific and technological advancements of various fields including chemical and electrochemical applications, the scope for exploration of nano-scale applications is still wide open. The intimate correlation between material properties and synthesis in combination with the urgency to enhance the empirical understanding of nanomaterials demand the evolution of new strategies to promising materials. Herein we introduce a rapid pyro-synthesis that produces highly crystalline functional nanomaterials under reaction times of a few seconds in open-air conditions. The versatile technique may facilitate the development of a variety of nanomaterials and, in particular, carbon-coated metal phosphates with appreciable physico-chemical properties benefiting energy storage applications. The present strategy may present opportunities to develop “design rules” not only to produce nanomaterials for various applications but also to realize cost-effective and simple nanomaterial production beyond lab-scale limitations.

One-Pot Synthesis of Multi-Morphous LiFePO4 Nanoparticles in Polyol Medium

Nano-LiFePO 4 possessing plate, rod and multi shaped morphologies are synthesized by a low temperature solvothermal route. The prepared samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. The XRD patterns indicate the formation of phase-pure orthorhombic olivine structure designated to the space group of Pnma. Synthesized samples reveal plate, rod and multi-morphous (a combination of plate, rod and spherical shaped particles) morphologies, respectively, in the SEM study. The growth of the plate-type olivine nanocrystals is identified to be along [100] and [010] crystallographic directions while the rod-shaped particles indicate growth along the crystallographic ac planes, as estimated from the TEM studies. The first discharge capacities of the olivine with plate-type, rod-type, and multi-morphous olivine samples are 131, 144, and 161 mAhg ―1 with minimal capacity fading for deeply extended cycles, respectively. Especially, in rate performance, the multi-morphous LiFePO 4 sample having mixed morphologies maintains capacity of about 110 mAh g ―1 until 8 C rates, while the plate and rod samples show severe capacity decline at corresponding rates. The reason for the high rate-performance of multi-morphous LiFePO 4 cathode is ascertained to the nano-sized particles and enhanced intimate connectivity between the multi-shaped nanoparticles.

SYNTHESIS OF HIGHLY CRYSTALLINE OLIVINE-TYPE LiFePO4 NANOPARTICLES BY SOLUTION-BASED REACTIONS

LiFePO4 nanocrystals were synthesized in various polyol media without any further post-heat treatment. The LiFePO4 samples synthesized using three different polyol media namely, diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TTEG), exhibited plate and rod-shaped structures with average sizes of 50–500 nm. The X-ray diffraction (XRD) patterns were indexed on the basis of an olivine structure (space group: Pnma). The samples prepared in DEG, TEG, and TTEG polyol media showed reversible capacities of 123, 155, and 166 mAh/g, respectively, at current density of 0.1 mA/cm2 with no capacity fading and exhibited excellent capacity retention up to the 50th cycle. In particular, the samples showed excellent performances at high rates of 30 and 60 C with high capacity retention. It is assumed that the nanometer size materials (~50 nm) possessing a highly crystalline nature may generate improved performance at high rate current densities.

Synthesis of LiMPO4 (M=Fe, Mn, Co) nanocrystals in polyol medium and their electrochemical properties

Olivine-structured LiMPO4 (M=Fe, Mn, Co) nanocrystals were synthesized by the solvothermal process in a polyol medium of diethylene glycol (DEG) at low temperature without any further post-heating process. The x-ray diffraction (XRD) patterns of LiMPO4 (M=Fe, Mn, Co) revealed highly crystalline and pure olivine structure (space group: Pnma) based on the Rietveld refinement method. The synthesized LiMPO4 (M=Fe, Mn, Co) nanocrystals with morphologies of plates and rods exhibited average particle sizes of 325, 487 and 581 nm, respectively. Among them, LiFePO4 nanocrystals showed an excellent reversible capacity of 167 mA h g−1 that corresponds to 98% utilization of its theoretical capacity and with a good extended cyclability rate of 50 C.

Low-temperature synthesis of LiFePO4 nanocrystals by solvothermal route

LiFePO4 nanocrystals were synthesized at a very low temperature of 170°C using carbon nanoparticles by a solvothermal process in a polyol medium, namely diethylene glycol without any heat treatment as a post procedure. The powder X-ray diffraction pattern of the LiFePO4 was indexed well to a pure orthorhombic system of olivine structure (space group: Pnma) with no undesirable impurities. The LiFePO4 nanocrystals synthesized at low temperature exhibited mono-dispersed and carbon-mixed plate-type LiFePO4 nanoparticles with average length, width, and thickness of approximately 100 to 300 nm, 100 to 200 nm, and 50 nm, respectively. It also appeared to reveal considerably enhanced electrochemical properties when compared to those of pristine LiFePO4. These observed results clearly indicate the effect of carbon in improving the reactivity and synthesis of LiFePO4 nanoparticles at a significantly lower temperature.

Polyol Synthesis of Pd/Ag Alloy Nanocrystalline

A solid solution of Pd-Ag nanoparticles was synthesized by the conventional polyol process. Silver nitrate (AgNO 3 , Aldrich), palladium(II) nitrate hydrate [Pd(NO 3 ) 2 ·xH 2 O, Aldrich], and ethylene glycol were used as the starting materials. Ethylene glycol was used as the surfactant for the solvent and reductant. Water was used to ensure that the reduction potentials of the two metals remained very similar in the aqueous solution. The obtained alloy nanopowder sample was characterized by X-ray diffraction (XRD). The obtained XRD pattern was refined using the Full-Prof program according to the Rietveld method and the mean crystallite sizes were estimated using the Scherrer equation. The quantitative atomic percentage across the nanoparticles was determined by using scanning transmission electron microscopy images to confirm the formation of the Pd/Ag alloy nanopowder.