Note: O – oxidation and R – reduction
The energy-dispersive X-ray spectroscopy (EDS) was conducted on anode and cathode active materials for examining sulfur content which is represented as the Pb/S ratio. The Pb/S is the ratio of composition obtained from the Pb, PbO2 and PbSO4peaks in EDS spectrum (Table 2). Higher values of Pb/S means lower SO42- content and vice versa. The Pb/S ratio indicative of the relative amount of sulfate (SO42-) getting deposited on anode and cathode after cycling. Pb/S ratio of the fresh cell is significantly different due to complete reversibility of PbSO4 to Pb as compared to that observed in cycled cells. As indicated from Pb/S ratio it can be seen that SO42-deposition and hardening were higher in anode and cathode in discharged state which eventually led to reduction in the amount of active materials with cycling (130 cycles).21,22,24 It can be seen that SO42- content increases with decreasing temperature from 40 to 0 °C due to slower kinetics and lower PbSO4 reversibility to Pb. However, it is lowest at -10 °C as compared to cycled and fresh cells essentially due to reduced active material (~50%) availability as seen from the charge-discharge characteristics (Figures 3-5).6,18,25