Soyeon Lee

The High Performance of Crystal Water Containing Manganese Birnessite Cathodes for Magnesium Batteries.

Rechargeable magnesium batteries have lately received great attention for large-scale energy storage systems due to their high volumetric capacities, low materials cost, and safe characteristic. However, the bivalency of Mg(2+) ions has made it challenging to find cathode materials operating at high voltages with decent (de)intercalation kinetics. In an effort to overcome this challenge, we adopt an unconventional approach of engaging crystal water in the layered structure of Birnessite MnO2 because the crystal water can effectively screen electrostatic interactions between Mg(2+) ions and the host anions. The crucial role of the crystal water was revealed by directly visualizing its presence and dynamic rearrangement using scanning transmission electron microscopy (STEM). Moreover, the importance of lowering desolvation energy penalty at the cathode-electrolyte interface was elucidated by working with water containing nonaqueous electrolytes. In aqueous electrolytes, the decreased interfacial energy penalty by hydration of Mg(2+) allows Birnessite MnO2 to achieve a large reversible capacity (231.1 mAh g(-1)) at high operating voltage (2.8 V vs Mg/Mg(2+)) with excellent cycle life (62.5% retention after 10000 cycles), unveiling the importance of effective charge shielding in the host and facile Mg(2+) ions transfer through the cathodes interface.

Direct Observation of an Anomalous Spinel-to-Layered Phase Transition Mediated by Crystal Water Intercalation

The phase transition of layered manganese oxides to spinel phases is a well-known phenomenon in rechargeable batteries and is the main origin of the capacity fading in these materials. This spontaneous phase transition is associated with the intrinsic properties of manganese, such as its size, preferred crystal positions, and reaction characteristics, and it is therefore very difficult to avoid. The introduction of crystal water by an electrochemical process enables the inverse phase transition from spinel to a layered Birnessite structure. Scanning transmission electron microscopy can be used to directly visualize the rearrangement of lattice atoms, the simultaneous insertion of crystal water, the formation of a transient structure at the phase boundary, and layer-by-layer progression of the phase transition from the edge. This research indicates that crystal water intercalation can reverse phase transformation with thermodynamically favored directionality.

Counting lithium ions in the diffusion channel of an LiV2O4 crystal

As a new microscopic method to reveal lithium ion behavior in lithium ion batteries, we demonstrated that lithium atoms in the diffusion channel of the spinel structure (LiV2O4 crystal) were visualized and their number was countable one-by-one by using annular bright field imaging method in conjunction with a spherical aberration corrected electron microscope: the lithium column intensity varied by a step of single lithium atom in correlation with the thickness change of the LiV2O4 crystal, in accordance with theoretical image simulations.

Reversible contrast in focus series of annular bright field images of a crystalline LiMn2O4 nanowire

A through-focus series of annular bright field (ABF) images were observed simultaneously with high-angle annular dark field (HAADF) images of very thin lithium manganese oxide (LiMn₂O₄), a typical cathode material used in lithium ion batteries, using a spherical aberration corrected electron microscope with a 50 pm resolution (R005). The ABF images showed dark dips at the positions of Li and Mn--O atomic columns, which reversed to bright peaks when the defocus sign was changed, as commonly observed in phase contrast images. The optimal defocus for the ABF images was about 2 nm of over-focus, while that for the HAADF images was 2 nm of under-focus for an incident probe with a convergent semi-angle of 30 mrad. These experimental results are interpreted based on a weak-phase-object approximation.

Phase transitions in a LiMn2O4 nanowire battery observed by operando electron microscopy.

Fast charge-discharge process has been reported to give a high capacity loss. A nanobattery consisting of a single LiMn2O4 nanowire cathode, ionic liquid electrolyte and lithium titanium oxide anode was developed for in situ transmission electron microscopy. When it was fully charged or discharged within a range of 4 V in less than half an hour (corresponding average C rate: 2.5C), Li-rich and Li-poor phases were observed to be separated by a transition region, and coexisted during whole process. The phase transition region moved reversibly along the nanowire axis which corresponds to the [011] direction, allowing the volume fraction of both phases to change. In the electron diffraction patterns, the Li-rich phase was seen to have the (100) orientation with respect to the incident electron beam, while the Li-poor phase had the (111̅) orientation. The orientation was changed as the transition region moved. However, the nanowire did not fracture. This suggests that a LiMn2O4 nanowire has the advantage of preventing capacity fading at high charge rates.

Electron microscopy at a sub-50 pm resolution

An aberration-corrected electron microscope developed in CREST project has been applied for imaging atoms and clusters buried inside crystals. The resolution of the microscope in scanning transmission electron microscopy (STEM) has experimentally proved to be better than 47 pm by use of a cold-field emission gun at 300 kV. The high resolution has given an advantage for imaging light elements such as lithium atoms discriminating one by one. Moreover, a three-dimensional structure imaging has been demonstrated for dopant clusters by a sub-50 pm STEM, using its high depth resolution.

Visualization of lithium ions by annular bright field imaging

The detection of lithium ions is required for characterization of lithium ion batteries, since the movement of lithium ions in the battery is one of the key ways to improve the performance. Annular bright field (ABF) imaging enables us to visualize individual lithium atomic columns simultaneously with heavy elements. Furthermore, it has been found that the number of lithium ions at the column is countable when the specimen is thin. These results suggest that movement of lithium ions in the material can be observed by taking consecutive ABF images during operation or in situ ABF observation. Actually, the spinel structure of L2V4O crystals was directly observed to be transformed into the defective NaCl structure at the moment when lithium ions were extracted from the original position during electron beam irradiation. We clarify the features of ABF imaging by comparing it with HAADF imaging in order to understand what information can be obtained by ABF imaging directly.

In-situ Annular Bright-Field Imaging of Structural Transformation of Spinel LiV2O4 Crystals into Defective LixV2O4

Thin LiV2O4 crystals with a spinel structure were observed by annular bright-field (ABF) imaging in which the contrast of lithium, vanadium, and oxygen columns varies depending on the number of ions in each column. On intense electron beam irradiation, lithium ions started to be displaced from the tetrahedral sites, which induced the redistribution of vanadium ions at octahedral sites. Consequently, the spinel structure was transformed into a defective NaCl structure. In-situ ABF imaging of thin specimens is a promising method for investigating local structural transformations accompanied by the displacement of lithium ions.

50pm Aberration Corrected In-situ Electron Microscopy - How Ion behaves in Lithium Ion Nanowire Battery

Lithium ion batteries (LIBs) attract much interest, and fundamental understanding of mechanisms enabling longer life for rapid charge-discharge and large capacity in use, particularly, of automobiles. Charge-discharge cycle causes structural phase transition [1] of electrode materials, and rapid cycle results generally in an irreversible cycle. Any irreversible change of the electrode structure can result in capacity fading and/or fracture of the electrode [2]. In order to study dynamical process of charge-discharge cycle, in-situ electron microscopy with electrochemical information has been devised to give rich results [3, 4].

In-situ STEM Observation of Strain Field Movement in a LiMn 2 O 4 Nanowire Battery

1. Quantum Nanoelectronics Research Center, Tokyo Institute of Technology, 2-12-1-H-51 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan. 2. JST–CREST, 7-gobancho, Chiyoda-ku, Tokyo 102-0075, Japan. 3. School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, 923-1292, Japan. 4. Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Umezono, 1-1-1, Tsukuba, 305-8568, Japan. 5. Department of Electronic Chemistry, Tokyo Institute of Technology, G1-1 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 6. Department of Physics, Tokyo Institute of Technology, 2-12-1-H-51 Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan.