Jeongbae Yoon

Discovery of abnormal lithium-storage sites in molybdenum dioxide electrodes

Developing electrode materials with high-energy densities is important for the development of lithium-ion batteries. Here, we demonstrate a mesoporous molybdenum dioxide material with abnormal lithium-storage sites, which exhibits a discharge capacity of 1,814 mAh g−1 for the first cycle, more than twice its theoretical value, and maintains its initial capacity after 50 cycles. Contrary to previous reports, we find that a mechanism for the high and reversible lithium-storage capacity of the mesoporous molybdenum dioxide electrode is not based on a conversion reaction. Insight into the electrochemical results, obtained by in situ X-ray absorption, scanning transmission electron microscopy analysis combined with electron energy loss spectroscopy and computational modelling indicates that the nanoscale pore engineering of this transition metal oxide enables an unexpected electrochemical mass storage reaction mechanism, and may provide a strategy for the design of cation storage materials for battery systems.

In Operando Monitoring of the Pore Dynamics in Ordered Mesoporous Electrode Materials by Small Angle X-ray Scattering.

To monitor dynamic volume changes of electrode materials during electrochemical lithium storage and removal process is of utmost importance for developing high performance lithium storage materials. We herein report an in operando probing of mesoscopic structural changes in ordered mesoporous electrode materials during cycling with synchrotron-based small angel X-ray scattering (SAXS) technique. In operando SAXS studies combined with electrochemical and other physical characterizations straightforwardly show how porous electrode materials underwent volume changes during the whole process of charge and discharge, with respect to their own reaction mechanism with lithium. This comprehensive information on the pore dynamics as well as volume changes of the electrode materials will not only be critical in further understanding of lithium ion storage reaction mechanism of materials, but also enable the innovative design of high performance nanostructured materials for next generation batteries.

Mesoporous transition metal dichalcogenide ME2 (M = Mo, W; E = S, Se) with 2-D layered crystallinity as anode materials for lithium ion batteries

Mesoporous transition metal dichalcogenides (TMDCs), composed of group VI metals (Mo and W) and chalcogens (S and Se), with 2-D layered crystalline frameworks and 3-D pore structures were successfully prepared via a melting-infiltration assisted nano-replication method using a mesoporous template KIT-6 with cubic Ia3d symmetry. Combined analysis using X-ray diffraction, N2 adsorption–desorption and electron microscopy indicated that the mesoporous TMDCs, thus obtained, exhibited high surface areas (87–105 m2 g−1), large pore volumes (0.21–0.25 cm3 g−1) and well-defined mesopores about 20 nm in diameters. The mesoporous TMDCs showed outstanding rate capabilities up to 2C as well as high reversible lithium storage capacities (MoS2 710 mA h g−1; MoSe2 744 mA h g−1; WS2 501 mA h g−1; WSe2 427 mA h g−1) without a remarkable fading of capacity.

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.

A Study on the Structural and Electrochemical Properties of Li 0.99 Ni 0.46 Mn 1.56 O 4 Cathode Material Using Synchrotron based in-situ X-ray Diffraction

The structural and electrochemical properties of Li0.99Ni0.46Mn1.56O4 (Fd m, disordered spinel) cathode material were studied and compared with stoichiometric LiNi0.5Mn1.5O4 (P4332, ordered spinel). First cycle discharge capacity of Li0.99Ni0.46Mn1.56O4 was similar to that of LiNi0.5Mn1.5O4 at C/3 and 1C rate, but cycling performance of Li0.99Ni0.46Mn1.56O4 was better than that of LiNi0.5Mn1.5O4 especially at high rate of 1C. This can be explained by performing synchrotron based in-situ XRD and results of GITT measurements. It is considered that faster lithium ion diffusion in the Li0.99Ni0.46Mn1.56O4 cathode results in the improvement of the rate capability. To study structural changes during cycling, synchrotron in-situ XRD patterns of both the samples were recorded at C/ 3 and 1C rate. Compared to stoichiometric LiNi0.5Mn1.5O4, disordered Li0.99Ni0.46Mn1.56O4 spinel sample has pseudo one phase behavior and one step phase transition between two cubic phases. So, LiNi0.5Mn1.5O4 would experience a much greater strain and stress, originating from the two phase transitions between three cubic phases and suffer from capacity loss during cycling especially at high rate.

Nanostructural Uniformity of Ordered Mesoporous Materials: Governing Lithium Storage Behaviors

Nanostructured materials make a considerable impact on the performance of lithium-storage characteristics in terms of the energy density, power density, and cycle life. Direct experimental observation, by a comparison of controlled nanostructural uniformity of electrode materials, reveals that the lithium-storage behaviors of mesoporous MoO2 and CuO electrodes are linearly correlated with their nanostructural uniformity. Reversible capacities of mesoporous MoO2 and CuO electrodes with well-developed nanostructures (1569 mA h g-1 for MoO2 and 1029 mA h g-1 for CuO) exceed their theoretical capacity based on the conversion reaction (838 mA h g-1 for MoO2 and 674 mA h g-1 for CuO). Given that exact understanding of the origin of the additional capacity is essential in maximizing the energy density of electrode material, this work may help to gain some insights into the development of high energy-density lithium-storage materials for next-generation lithium rechargeable batteries.