Hideki Oki

First‐Principles Calculations of Lithium‐Ion Migration at a Coherent Grain Boundary in a Cathode Material, LiCoO2

Results of theoretical calculations are reported, examining the effect of a coherent twin boundary on the electrical properties of LiCoO(2) . This study suggests that internal interfaces in LiCoO(2) strongly affect the battery voltage, battery capacity, and power density of this material, which is of particular concern if it is used in all-solid-state Li-ion batteries.

Real-time direct observation of Li in LiCoO2 cathode material

The direct observation of light elements such as Li is a challenge even for state-of-the-art electron microscopy techniques because such elements scatter electrons only weakly. Using the annular bright field scanning transmission electron microscopy imaging technique, we have simultaneously visualized columns of Li, O, and Co ions in the lithium-ion battery cathode material LiCoO2, which is one of the most important cathode materials for industrial applications. The annular bright field image exhibits a good signal-to-noise ratio and the image contrast is not reversed as the specimen thickness changes. The direct visualization of light elements in real time with this method represents an important breakthrough in characterizing the active materials in solid-state electrochemical devices.

Domain boundary structures in lanthanum lithium titanates

Perovskite-type lanthanum lithium titanate (LLTO) is attracting extensive interest because of its high intrinsic ionic conductivity. The material exhibits a complex microstructure with domains of various sizes and orientations that vary with the lithium content. Based on a systematic examination of both Li-poor and Li-rich LLTO compounds using state-of-the-art scanning transmission electron microscopy (STEM), we reveal the structures and composition of the domain boundaries (DBs) and consider their effect on Li-ion mobility and ionic conductivity, in the process positing the origin of the microstructural variations. DBs in this material are shown to consist essentially of two types: frequently occurring 90° rotation DBs and a much less common antiphase-type boundary. It is found that the 90° DBs are coherent interfaces consisting of interconnected steps that share La sites, with occupancies of La sites higher than in the domain interiors. The origin of microstructural variations in the two compounds is associated with different degrees of lattice mismatch strain at DBs in Li-poor and Li-rich materials. The lattice strain and associated O vacancies, as well as the high La occupancies, at DBs are expected to result in lower interdomain Li-ion mobility, which will have a deleterious effect on the overall ion conductivity.

Preparation of Layered-Rhombohedral LiCoO2 Epitaxial Thin Films Using Pulsed Laser Deposition

Epitaxial thin films of layered-rhombohedral LiCoO2 (α-NaFeO2 structure, R3m) have been successfully grown on Al2O3(0001) substrates using pulsed laser deposition. A single phase of LiCoO2 was obtained in the narrow substrate temperature range of 250–300 °C, above which secondary phases, such as Co2O3, Co3O4, and LiCo2O4, appeared. In addition, it was found that annealing of precursor films deposited at room temperature yielded atomically flat LiCoO2 films with a surface roughness of ~0.2 nm.

The influence of charge ordering on the phase stability of spinel LiNi2O4

The transition metal ions in spinel LiNi2O4 are commonly assumed to be in a single charge state and uniformly distributed throughout the crystal lattice. However, multivalent Ni can exist in a number of different charge states simultaneously within the crystal, whilst still maintaining overall charge neutrality. By carrying out a systematic study of possible charge states of Ni and their distribution in LiNi2O4 using first-principles calculations within the framework of density functional theory, we show that two charge-ordered phases are more stable than the charge-disordered spinel phase: (a) an orthorhombic phase containing Ni3+ and Ni4+ ions in a 1 : 1 ratio resulting in Imma symmetry, and (b) a cubic phase with Ni2+ and Ni4+ in a 1 : 3 ratio with P4332 symmetry. Our results indicate that charge ordering is an important process in stabilizing the spinel phase of LiNi2O4, and should not be ignored in any quantitative study. Our findings are of particular importance for elucidating the relative stabilities of the spinel phase and layered phase of Li1−xNi2O4, a delithiated form of lithium secondary battery cathode material LiNiO2.