A serial interface interposed between two serially connected unit cells provides mechanical strength to the serial connection and conductive coupling therebetween. The serial interface includes an inter-connector having a first vent hole formed therein configured for conductively coupling an anode outer wall of a first unit cell and a cathode terminal of a second unit cell to each other; and a spacer having a second vent hole formed therein and configured to be disposed between the inter-connector and an anode outer wall of the second unit cell to prevent a short-circuit due to the movement of the inter-connector.