First Cross-Section Observation of an All Solid-State Lithium-Ion “Nanobattery” by Transmission Electron Microscopy

Understanding and improving the behavior of interfaces is essential to the development of safer and high performance Li-based batteries regardless of their range of applications. Indirect methods such as impedance spectroscopy or direct methods such as the live in situ observation of batteries cycled within a scanning electron microscope (in situ SEM) are used to determine the interface microstructure/composition evolution upon cycling. These methods are used to establish a direct link between interface properties and batteries performance; they also enable us to spot local interface defects that are crucial to the development of 2D solid-state microbattery, for instance. Indeed, this technology is of interest in powering the new generation of microelectromechanical systems (MEMS). Here, we demonstrate the first ex situ TEM observation of "nanobatteries" obtained by cross-sectioning a microbattery using focus ion beam (FIB) in a dual beam SEM. Then, TEM analyses between pristine, cycled, and faulted all solid-state LiCoO2/solid electrolyte/SnO Li-ion batteries have revealed drastic changes such as the presence, depending on the battery fabrication process, of both cavities within the solid electrolyte layers and low wetting points between the electrolyte and the negative electrode. Moreover, post-mortem TEM observations of cycled microbatteries have revealed a rapid deterioration of the interface upon cycling because of the migration of the chemical elements between stacked layers. Such findings are involved both in the improvement of the reliability of the 2D all solid-state battery assembling process and in the enhancement of their cycling performances. Such achievements constitute the technical platform for our future targets namely the development of live in situ TEM observation of "nanobatteries" cycled within the microscope.