In the past 5 years, we have witnessed an increased interest in developing secondary aluminum–sulfur (Al–S) batteries. One of the main obstacles to further advance this technology is the large overpotentials observed during the recharge of the battery,which to a large extent are related to the insulating nature of the discharge product, namely Al2S3. The present work aims to elucidate the mechanisms responsible for ionic and electronic transport in Al2S3 and their eventual connection with the experimentally observed overpotentials. We use density functional theory at the hybrid functional level to determine the concentrations and mobilities of a large set of potential charge carriers in thesystem. We found that despite decent mobilities, all carriers exhibit negligible concentrations at equilibrium potential and thus cannot contribute to the conductivity. Nevertheless, we estimate that a 1V overpotential (a value very close to that observed experimentally) exponentially raises the presence of negatively charged interstitial sulfur ions to the point that a practical ionic conductivity is reached. Our study points out the importance of designing strategies to increase the concentrations of charge carriers in Al2S3 to reduce the overpotentials in Al–S batteries.