Tsukasa Hirayama

Electric shielding films for biased TEM samples and their application to in situ electron holography

We developed a novel sample preparation method for transmission electron microscopy (TEM) to suppress superfluous electric fields leaked from biased TEM samples. In this method, a thin TEM sample is first coated with an insulating amorphous aluminum oxide (AlOx) film with a thickness of about 20 nm. Then, the sample is coated with a conductive amorphous carbon film with a thickness of about 10 nm, and the film is grounded. This technique was applied to a model sample of a metal electrode/Li-ion-conductive-solid-electrolyte/metal electrode for biasing electron holography. We found that AlOx film with a thickness of 10 nm has a large withstand voltage of about 8 V and that double layers of AlOx and carbon act as a nano-shield to suppress 99% of the electric fields outside of the sample. We also found an asymmetry potential distribution between high and low potential electrodes in biased solid-electrolyte, indicating different accumulation behaviors of lithium-ions (Li+) and lithium-ion vacancies (VLi-) in the biased solid-electrolyte.

Quantitative Operando Visualization of Electrochemical Reactions and Li Ions in All-Solid-State Batteries by STEM-EELS with Hyperspectral Image Analyses

All-solid-state lithium-ion batteries (LIBs) are one of the promising candidates to overcome some issues of conventional LIBs with liquid electrolytes. However, high interfacial resistance of Li-ion transfer at the electrode/solid electrolyte limits their performance. Thus, it is important to clarify interfacial phenomena in a nanometer scale. Here, we present a new method to dynamically observe the Li-ion distribution and Co-ion electronic states in a LiCoO2 cathode of the all-solid-state LIB during charge and discharge reactions using operando scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). By applying a hyperspectral image analysis of non-negative matrix factorization (NMF) to the STEM-EELS, we succeeded in clearly observing the quantitative Li-ion distribution in the operando condition. We found from the operando observation with NMF that the Li ions did not uniformly extract/insert during the charge/discharge reactions, and the activity of the electrochemical reaction depended on the Li-ion concentration in a pristine state. An electrochemically inactive region was formed about 10-20 nm near the LiCoO2/Li2O-Al2O3-TiO2-P2O5-based solid electrolyte interfaces. The STEM-EELS, electron diffraction, and Raman spectroscopy experimentally showed that the inactive region was a mixture of LiCoO2 and Co3O4, leading to the higher interfacial resistance of the Li-ion transfer because Co3O4 does not have pathways of Li-ion diffusion in its crystal.

Nanostructural Characterization of Low Resistance Joints Using Ag Pastes for GdBa2Cu3O7-x Coated Conductors

GdBa2Cu3O7-x coated conductors were splice jointed by a face-to-face manner using a paste containing nano-sized Ag particles under a pressure of about 50 MPa at 150 °C for 1 hr. The low electrical resistance of 6 nΩ at the joint was attained. Nanostructural characterizations of the starting Ag paste and the jointed region of the coated conductors were carried out using scanning electron microscopy and transmission electron microscopy. The size of the Ag particles in the starting pastes were confirmed to be a few tens of nanometers in diameter. The size of Ag particles became larger during the jointing process. Both the surfaces of the stabilizing Ag layers were partially bonded by the Ag particles. No oxides or other elements were detected in the region of the bonding parts.

Electric Potential Profiling of an All-solid-state Lithium Ion Battery by In situ Electron Holography

Lithium-ion batteries, which provide the largest energy storage densities among several battery technologies, can serve as storage devices for renewable energy. Therefore, they are considered an essential technology for environmentally friendly and sustainable societies [1]. However, the electrochemical reactions in the batteries that control their performance are not yet fully understood. Consequently, important clues to develop more efficient batteries are hard to find. One effective way to understand electrochemical reactions in the batteries is to visualize electric potential distributions inside them, in particular, near the interface between an electrode and an electrolyte during chargedischarge cycling.