Jae-Hyun Shim

As a prominent next-generation anode material for high-capacity applications, silicon stands out due to its potential. Crystalline silicon, which offers a higher initial capacity compared to its amorphous counterpart, presents challenges in practical applications due to its poor cycling performance. In this study, we prepared composites of crystalline and amorphous silicon with graphite, assembled pouch-type full cells, and evaluated their suitability for practical use. The material incorporating amorphous silicon demonstrated superior performance at both high and low rates, as well as various temperatures. Additionally, the changes in cell thickness during charge and discharge, i.e., the volume changes in the anode material, are significantly related to cycling performance. We examined the microscopic interactions between silicon and lithium atoms using molecular dynamics simulations. Our observations indicate that lithium migration within amorphous silicon, which has lower activation energy, is much easier than in crystalline silicon. In crystalline silicon, lithium penetration is greatly influenced by the orientation of the crystal planes, resulting in anisotropic volume expansion during lithiation.

The development of novel anode materials for high energy density is required. Alloying Si with other metals is a promising approach to utilize the high capacity of Si. In this work, we optimized the composition of a Si–Ti–Al ternary alloy to achieve excellent electrochemical performance in terms of capacity, cyclability, and rate capability. The detailed internal structures of the alloys were characterized through their atomic compositions and diffraction patterns. The lithiation process of the alloy was monitored using real-time scanning electron microscopy, revealing that the mechanical stability of the optimized alloy was strongly enhanced compared to that of the pure silicon material.

Investigating the structural properties of Li-ion batteries (LIBs) and, if possible, determining the overall structure is important in understanding the physical and chemical properties. Understanding the structural properties and the electrochemical properties of the arranged atoms of cathode materials is therefore essential for new battery material design and system optimization. Particularly, cathode materials such as LiNi_xMnyCo_1-x-yO₂ (NMC), lithium-rich Li(Li_yNi_x-yMn_xCo_1-2x)O₂, lithium-rich / manganese-rich xLi₂MnO₃ (1-x)LiMO₂ are analyzed by the scanning transmission electron microscopy (STEM) techniques to identify structural characteristics, leading to the conversion of the layer structure to the spinel and/or salt structure, leading to increased impedance, voltage reduction and capacity of the LIBs. The atomic behavior of Li ion in the cathode materials that determines LiBs performance is hardly characterized by transmission electron microscopy (TEM) owing to its weak electron-scattering power. In this sense, annular bright-field (ABF) STEM, in which the contrast has a low scaling rate with the atomic number, has been proven to be a robust technique for simultaneous imaging of light and heavy elements. However, it is very important to understand the damage mechanism of cathode materials by electron-beam irradiation in STEM and to develop this technique. In this study, we investigated the effect of electron-beam irradiation damage on the surface reaction layer and LiNi_0.6Co_0.2Mn_0.2O₂ material during STEM data acquisition. This electron-beam irradiation damage process is similar to the lattice reconstruction and chemical evolution caused by the charge-discharge cycle. Furthermore, based on the phase transition phenomenon by the electron-beam irradiation, it is examined whether the electron beam irradiation damage in the lattice structure substituted by Al and Ti affects the phase transition phenomenon.

The electrochemical properties of Li(NixCoyMnz)O₂ (NCM) materials are decisively determined by the interplay of combined disordering of cations and the relevant chemical/electronic changes.