EELS Studies of materials for solid-state battery - On the limitation of TEM/STEM toward quantification
Department of Physics
Arizona State University
Tempe, AZ 85287-1504
Radiation damage remains an important obstacle to extend applications of (scanning) transmission electron microscopy (TEM/STEM). Aberration correction allows the STEM objective aperture (condenser aperture in TEM nanodiffraction mode) to be enlarged so that the electron probe on the specimen may have a very high current density, e.g. > 106 A/cm2. This value is about 104 – 105 times larger than the current density used in forming conventional HREM images, and about 106 – 107 larger than that used for bright-field diffraction contrast imaging. Can materials survive under these conditions?
The materials for solid-state batteries are very good conductors of ions but are essentially insulating toward electrons. These materials usually contain mobile ions such as Ag+, Li+, Na+ or H+ for conduction. These features ensure that they are all susceptible to high-energy electron irradiation, and can be damaged or modified easily by the electron beam. But how serious this problem could be and how to avoid it? Based on extensive electron energy-loss spectroscopy (EELS) studies in various Li compounds, we are able to identify the dominant mechanism responsible for their susceptibility to electron beam in this group of materials. The commonly recognized knock-on and radiolysis play only a minor role, compared with the electron irradiation induced electric field. Driven by the induced electric field, the damages vary from the long-range Li migration over a distance larger than several microns to the short-range migration leading to the relocation of Li in different lattice sites. The commonly used ways to eliminate or reduce damage, such as lowering beam voltage, lowering specimen temperature and coating with protection films, may not help due to the nature of damage mechanism. Instead, a feasible approach within the currently available technologies is lowering the beam current, but giving up spatial resolution perhaps.