The present disclosure is directed to an energy storage device having improved thermal performance. More specifically, the energy storage device includes a housing with side walls that define an internal volume. The side walls include bottom and front side walls, with the front side wall having an air inlet and outlet configured to circulate cooling air therethrough. The energy storage device also includes a plurality of cells arranged in a matrix within the internal volume atop the bottom side wall. Further, the cells define a top surface. Further, the energy storage device includes an exhaust manifold adjacent to the front side wall between at least a portion of the cells and the air inlet. Thus, the exhaust manifold is configured to direct airflow from the top surface towards the bottom side wall and then to the air outlet so as to provide an airflow barrier between cooling air entering the air inlet and the cells.