Ultra-low-voltage observation of battery materials by scanning electron microscopy

The mechanism of voltage contrast formation under ultra-low landing energy condition is discussed, by which a binder contained in lithium-ion battery anode material has been visualized with high contrast. Since the anode material is a complex experimental system with multiple contrast formation factors, a standard sample simulating it was fabricated for simplification. The binder was observed to be darker than the substrate at landing energies of 30-50 eV. The binder exhibited a distinct appearance reflecting its shape (in the 3D-particle mode) at 20 eV. The mirroring phenomenon occurred at 10 eV, in which the primary electrons bounced off the sample before irradiating the surface. The surface potential at the electron beam irradiation moment was presumed to affect the contrast formation, but direct measurement of it was difficult. Thus, the sample was transferred to an Atomic Force Microscope without exposure to the atmosphere to measure the 'residual' potential of the binder in KPFM mode after the SEM observations. Under darker binder observed conditions of 30-50 eV, KPFM measured residual potential was positive relative to the substrate. Under conditions of the 3D-particle mode at 20  eV and the mirroring phenomenon at 10 eV, the residual potentials were negative. Therefore, a correlation between the behavior of the voltage contrast and the residual potential was obtained. Finer landing-energy step measurement revealed hysteresis responses of voltage contrast and the residual potential to the landing energy. The Cause of the hysteresis was discussed.