Donghan Kim

Synthesis of LiFePO4 Nanoparticles in Polyol Medium and Their Electrochemical Properties

LiFePO 4 nanoparticles were synthesized using the polyol process without any further heating as a postprocessing step. The X-ray diffraction patterns of the sample exhibited a good fit with the orthorhombic phase with no unwanted impurity phases. The LiFePO 4 nanoparticles showed a reversible capacity of 166 mAh/g, which amounts to a utilization efficiency of 98%, with an excellent reversibility in extended cycles. The electrode shows an excellent capacity retention at high-rate current densities due to its single-crystal-like and monodispersed uniform morphology with orthorhombic shape with an average width of 20 nm and length of 50 nm.

A new synthesis route to nanocrystalline olivine phosphates and their electrochemical properties.

LiFePO4 nanoparticles were synthesized in various polyol mediums without any further heating. The LiFePO4 samples synthesized in polyol mediums exhibited average sizes of 20, 20, 50, and 50 nm with orthorhombic-like shapes. The XRD patterns were indexed on the basis of an olivine structure (space group : Pnma) except for the sample prepared in EG polyol medium. The LiFePO4 samples prepared in EG, DEG, TEG, and TrEG polyol mediums show the reversible capacity of 120 mA h/g, 144 mA h/g, 159 mA h/g, and 167 mA h/g at current density of 0.1 mA/cm2 with no capacity fading and excellent cycle retentions during extended cycles. Especially, the samples showed the excellent performances at high rate of 30 C and 60 C with high capacity retention. It is a speculation that nanometer size materials (approximately 50 nm) and a uniform size-distribution with highly crystallined phase may affect the excellent performances at high rate current densities.

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.

An in-situ gas chromatography investigation into the suppression of oxygen gas evolution by coated amorphous cobalt-phosphate nanoparticles on oxide electrode

The real time detection of quantitative oxygen release from the cathode is performed by in-situ Gas Chromatography as a tool to not only determine the amount of oxygen release from a lithium-ion cell but also to address the safety concerns. This in-situ gas chromatography technique monitoring the gas evolution during electrochemical reaction presents opportunities to clearly understand the effect of surface modification and predict on the cathode stability. The oxide cathode, 0.5Li2MnO3∙0.5LiNi0.4Co0.2Mn0.4O2, surface modified by amorphous cobalt-phosphate nanoparticles (a-CoPO4) is prepared by a simple co-precipitation reaction followed by a mild heat treatment. The presence of a 40 nm thick a-CoPO4 coating layer wrapping the oxide powders is confirmed by electron microscopy. The electrochemical measurements reveal that the a-CoPO4 coated overlithiated layered oxide cathode shows better performances than the pristine counterpart. The enhanced performance of the surface modified oxide is attributed to the uniformly coated Co-P-O layer facilitating the suppression of O2 evolution and offering potential lithium host sites. Further, the formation of a stable SEI layer protecting electrolyte decomposition also contributes to enhanced stabilities with lesser voltage decay. The in-situ gas chromatography technique to study electrode safety offers opportunities to investigate the safety issues of a variety of nanostructured electrodes.