Multi-layered graphene based spin valves were deposited on SiO2/Si substrates and the Co ferromagnetic and non-magnetic electrodes were patterned using electron-beam lithography. Aberration-corrected (scanning) transmission electron microscopy imaging and energy dispersive X-ray spectroscopy were employed to study the interfacial structure of the device components from thin cross-sectional lamellae obtained by focused ion beam microscopy. The robustness of the spin injection with a constant spin polarization under positive and negative bias voltage was attributed to the interface smoothness which leads to suppression of spin scattering at the interface between the ferromagnet and the multi-layered graphene.