A novel BaTiO3-Na0.5Bi0.5TiO3-Nb2O5-NiO (BT-NBT-Nb-Ni) system that meets the X8R specification (-55 degrees C-150 degrees C , Delta C/C <=+/- 15%) of multilayer ceramic capacitors (MLCCs) was fabricated, with a maximum dielectric constant of 2350 at room temperature (25 degrees C). Core-shell microstructure was observed by transmission electron microscopy (TEM), accounting for the good dielectric temperature stability. The role of Ni on the formation of core-shell structure and phase structure, and the subsequent relationship between structure and dielectric/ionic conduction properties were investigated. It was observed that the addition of Ni could adjust the ratio of core/shell, and significantly reduces the dielectric loss over the studied temperature range. A new Ba11(Ni, Ti)(28)O66+x phase with a 10-layer close-packed structure was identified by X-ray diffraction (XRD), serving as a source of oxygen vacancies for ionic conduction in addition to Ba(Ni,Ti)O-3. Furthermore, the impedance spectroscopy measurements demonstrated the remarkable impact of these Ni-induced oxygen vacancies on both the grain and grain-boundary conductivities.