The self-discharge increases as the charging capacity of the positive AC electrode is increased (Fig. and Supplementary Fig. ). Independent of the positive electrode loading, already after a few hours resting time, the discharge capacities approached a similar level. This is also shown by charging to three different capacities (219, 330 and 441 mAh gC -1 ) and waiting for 4 h at OCV (Supplementary Fig. ). After 32 h of resting, the discharge capacities reached a value of 150 mAh gC -1 . Assuming that most capacity is stored in the form of solid I2 and by considering the known electrode porosities (Supplementary Table ) this corresponds to an I2 pore occupancy of 0.17. The proposed reaction mechanism explains the increasing self-discharge with increasing amounts of iodine loading/charging capacities. The more iodine is deposited, the more mobile I3 -and I5 -are formed chemically.