Stannous sulfate is commonly used as an additive in the positive lead pastes of lead-acid batteries, but its real function and mechanism are still vague and need further study. The present work investigates the intrinsic effects of tin additives on the positive electrode performance under well-controlled experimental conditions. The tin additives are added in three ways: the SnSO 4 dissolved in 4.5 M H 2 SO 4 electrolyte, the metal Sn melted in metal Pb substrate, and the SnSO 4 mixed in homemade industrial lead pastes. A series of evolutions are clearly revealed in structure, composition, and redox activity of the positive active materials with the content of Sn. Both the SnSO 4 in the electrolyte and in the positive lead pastes have a significant inhibiting effect on the transformation between PbSO 4 and PbO 2 , leading to a decrease in the charge/discharge capacities with increasing content of SnSO 4 . A similar but weaker inhibiting effect was also found for the Sn melted in the Pb-Sn alloys. The SnSO 4 has a significant enhancement effect on the grain size of PbSO 4 /PbO 2 , which increases the resistance of the solid-phase transformation deep inside the grains during the charge/discharge. All these results do not support the use of SnSO 4 in the positive pastes of lead-acid batteries. Graphical abstract