Donghyuk Jang

Exceptional electrochemical performance of freestanding electrospun carbon nanofiber anodes containing ultrafine SnOx particles

SnOx–carbon nanofiber (CNF) composites are synthesized using the electrospinning technique for use as freestanding electrodes in Li-ion batteries. The electrodes made from the composites carbonized at 750 °C (SnOx–CNF-750) with 14.5 wt% SnOx deliver a remarkable capacity of 674 mA h g−1 after 100 cycles when discharged at 0.5 A g−1. This result is considered the highest among those reported in the literature for anodes made from similar electrospun carbon fibers containing SnOx nanoparticles. An increase in carbonization temperature to 950 °C (SnOx–CNF-950) results in a significant reduction of the particle content in the fiber due to aggregation of Sn to form nanoparticles external to the fibers, with concomitant degradation of capacities. The presence of amorphous SnOx particles at the atomic scale embedded in the conductive CNFs is thought to be responsible for the exceptional electrochemical performance of the SnOx–CNF-750 electrodes. These ultrafine particles facilitate the reaction Sn + xLi2O → SnOx + 2xLi+ + 2xe−, making it highly reversible, which is confirmed by the growing peak currents with increasing scan rate indicated by cyclic voltammetry, and the absence of Sn–Sn bonds in the particles revealed by the extended X-ray absorption fine structure spectroscopy (EXAFS). Both the SnOx particle size and content in the fiber play important roles in controlling the rate and cyclic performance of the SnOx–CNF composite electrodes.

Crystal Structure Changes of LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials During the First Charge Investigated by in situ XRD

The structural changes of Li1-xNi0.5Co0.2Mn0.3O2 cathode material for lithium ion battery during the first charge was investigated in comparison with Li1-xNi0.8Co0.15Al0.05O2 using a synchrotron based in situ X-ray diffraction technique. The structural changes of these two cathode materials show similar trend during first charge: an expansion along the c-axis of the unit cell with contractions along the a- and b-axis during the early stage of charge and a major contraction along the c-axis with slight expansions along the a- and b-axis near the end of charge at high voltage limit. In Li1-x Ni0.5Co0.2Mn0.3O2 cathode, however, the initial unit cell volume of H2 phase is bigger than that of H1 phase since the c-axis undergo large expansion while a- and b- axis shrink slightly. The change in the unit cell volume for Li1-xNi0.5Co0.2Mn0.3O2 during charge is smaller than that of Li1-x Ni0.8Co0.15Al0.05O2. This smaller change in unit cell volume may give the Li1-xNi0.5Co0.2Mn0.3O2 cathode material a better structural reversibility for a long cycling life.

Structural and Electrochemical Properties of Doped LiFe 0.48 Mn 0.48 Mg 0.04 PO 4 as Cathode Material for Lithium ion Batteries

The electrochemical properties of Mg-doped and pure olivine cathodes are examined and the lattice parameters are refined by Rietveld analysis. The calculated atomic parameters from the refinement show that doping has a significant effect in the olivine structure. The unit cell volume is 297.053(2) for pure and is decreased to 296.177(1) for Mg-doped sample. The doping of cation with atomic radius smaller than and ion induces longer Li-O bond length in octahedra of the olivine structure. The larger interstitial sites in octahedra facilitate the lithium ion migration and also enhance the diffusion kinetics of olivine cathode material. The sample with larger Li-O bond length delivers higher discharge capacities and also notably increases the rate capability of the electrode.

Further utilization of a Mn redox reaction via control of structural disorder in olivine systems

Olivine-type phosphates have become cathode materials of great interest in Li rechargeable batteries because of their low cost, high energy density and thermal safety. Strategies such as introduction of non-stoichiometric character, doping, carbon coating, and reduction of particle size have been shown to improve the electrochemical performances of olivine materials. However, the underlying mechanism responsible for electrochemical enhancement by introduction of non-stoichiometric character and doping is not yet well understood at the structure level. In this study, we investigated the structural and electrochemical properties of the equi-sized LiFe0.5Mn0.5PO4, non-stoichiometric LiFe0.5−xMn0.5−xPO4−σ and Cr-doped LiFe0.5−xMn0.5−xCryPO4−σ materials in depth by using synchrotron X-rays, neutrons and electrochemical techniques in order to explore the underpinning science responsible for improved electrochemistry of olivines as a result of non-stoichiometry and supervalent doping. Our neutron diffraction study revealed that anti-site defects are a critical factor for improving the electrochemical performance of olivines, and these defects can be decreased by introducing non-stoichiometry in the crystal structure. The X-ray absorption near edge structure results show that the improved electrochemical performance obtained in non-stoichiometric LiFe0.5−xMn0.5−xPO4−σ and Cr-doped LiFe0.5−xMn0.5−xCryPO4−σ is achieved by a selective further oxidation of Mn, and there is no effect of non-stoichiometry and doping on the Fe2+/Fe3+ redox couple.