Because of the tremendous demanding for energy storage in electric vehicles and electronic consumable devices, lithium batteries have become the most important candidates, which strongly urge effective characterization methods, particularly to observe lithium ions at atomic scale. The recent success in aberration-corrected annular-bright-field (ABF) scanning transmission electron microscopy (STEM) provides a feasible access to a direct interpretation of the atomic structure at sub-angstrom resolution. Such a procedure has been proved to be highly efficient in resolving atomic columns of lithium as shown in our previous work ^[1-8].

Figure 1 shows a schematic of the ABF geometry with a convergent beam and an annular-shaped bright-field detector. A fine probe with a spot size less than 1 angstrom scans across the specimen with the annular detector defining a collection semiangle at given camera lengths. An ABF electron micrograph of LiFePO₄ viewed along [010] orientation is displayed in Fig. 1. It is demonstrated that not only Fe and P are revealed in this micrograph, but also the atomic positions of O and Li. The ABF contrast tends to minimize the variance of the atomic number by following a Z^1/3 dependency.

With the latest ABF method, we have further studied possible electrochemical reaction mechanisms and detailed structure evolution for (partially) lithiated / delithiated 1D-LiFePO₄, 2D- LiCoO₂, Li₂MnO₃ and 3D- Li₄Ti₅O₁₂ and other lithium-based active materials, which are essential for basic understanding and further material design. In addition, The ABF method can be further extended for atomic-scale investigation of oxygen vacancies and other structure evolution of light atoms.These shed new insight into the lithium / sodium storage mechanism in important cathode materials for Li- / Na- ion batteries.