Inhibiting effect of tin additives on the solid-phase conversion of PbSO4 to PbO2

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 SnSO4 dissolved in 4.5 M H2SO4 electrolyte, the metal Sn melted in metal Pb substrate, and the SnSO4 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 SnSO4 in the electrolyte and in the positive lead pastes have a significant inhibiting effect on the transformation between PbSO4 and PbO2, leading to a decrease in the charge/discharge capacities with increasing content of SnSO4. A similar but weaker inhibiting effect was also found for the Sn melted in the Pb-Sn alloys. The SnSO4 has a significant enhancement effect on the grain size of PbSO4/PbO2, 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 SnSO4 in the positive pastes of lead-acid batteries.