The separator has the function of a barrier that electrically isolates the two electrodes from each other to prevent internal short circuits. At the same time, however, the separator must be permeable to ions so that the electrochemical reactions can take place in the cell.

A separator must be thin so that the internal resistance is as low as possible and a high packing density can be achieved. This is the only way to achieve good performance data and high capacities.

Other important functions of the separator are to absorb the electrolyte and to ensure gas exchange in closed cells. While fabrics and paper were used in the past, very fine-pored materials such as nonwovens and membranes are predominantly used today.