A lithium ion battery module includes a battery cell stack disposed within a housing of the battery module. The stack includes a first battery cell, a second battery cell positioned adjacent to the first battery cell, and a battery cell separator fitted over the first battery cell. The battery cell separator includes a plurality of walls formed from a continuous material and defining a pocket in which the first battery cell is disposed. The plurality of walls is configured to electrically insulate the first cell from the second cell. The separator also includes a projection extending from a wall of the plurality of walls, the projection is positioned between a terminal of the first battery cell and a terminal of the second battery cell and is configured to electrically insulate the terminals from one another.