An energy storage device includes a first conductor having a first surface and a second surface. The energy storage device also includes a second conductor and a separator assembly that encloses the first conductor and that is disposed between the first and second conductors. The separator assembly also includes a first portion that covers the first surface and a second portion that covers the second surface. The first and second portions are attached to one another, and at least one of the first and second portions includes a first sheet and a second sheet that are attached to one another. The first sheet includes a first material, and the second sheet includes a second material that is different from the first material.