Analytical ABF-STEM imaging of Li ions in rechargeable batteries

Rechargeable batteries are being intensively investigated in an attempt to solve the energy issues while meeting the environmental demands. Even though Li-ion batteries (LIB) with high energy and light weight have been commercialized within the last 20 years, these devices currently require higher energy density, output power and sustainability characteristics. The atomic behavior of Li ion that determines LIB's performance is hardly characterized by transmission electron microscopy (TEM) owing to its weak electron-scattering power. In this sense, annular bright-field (ABF) scanning TEM (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. The s-state model, in which electron channeling along the atomic column allows the intensity to be focusing in the forward direction, has successfully explained the theory of ABF contrast. Furthermore, the detector angle range, the defocus-thickness dependence and the accelerating voltage (among other parameters) were discussed for optimized imaging conditions. ABF-STEM has shown powerful capabilities in resolving the atomic structure and the chemistry of electrodes (e.g. Li-ion occupation and diffusion, phase transformation and interface reaction), thereby providing critical insights into the physical properties, the battery performance and the design guidance of LIB. The future directions of ABF imaging for the characterization of LIB materials were also reviewed.