During sintering of Ni-electrode multi-layer ceramic capacitors (MLCCs), the Ni electrode often becomes discontinuous because of its lower sintering temperature relative to that of $BaTiO_3$. In an attempt to retard the sintering of Ni, we introduced passivation of the Ni powder. To find the optimal passivation conditions, a thermogravimetric analysis (TGA) was conducted in air. After passivation at $250^{circ}C$ for 11 h in air, a nickel oxide shell with a thickness of 4-5 nm was formed on nickel nanoparticles of 180 nm size. As anticipated, densification of the compacts of the passivated Ni/NiO core-shell powder was retarded: the starting temperature of densification increased from ${sim}400^{circ}C$ to ${sim}600^{circ}C$ in a $97N_2-3H_2$ (vol %) atmosphere. Grain growth was also retarded during sintering at temperatures of 750 and $1000^{circ}C$. When the sintering atmosphere was changed from wet $99.93N_2-0.07H_2$ to wet $99.98N_2-0.02H_2$, the average grain size decreased at the same sintering temperature. The conductivity of the passivated powder sample sintered at $1150^{circ}C$ for 8 h in wet $99.93N_2-0.07H_2$ was measured to be $3.9{ imes}10^4S/cm$, which is comparable with that, $4.6{ imes}10^4S/cm$, of the Ni powder compact without passivation. These results demonstrate that passivation of Ni is a viable means of retarding sintering of a Ni electrode and hence improving its continuity in the fabrication of $BaTiO_3$-based multi-layer ceramic capacitors.