Figure 9: (a) Anode, and (b) cathode in fully charged condition after 130 cycles, (c) anode and (d) cathode in discharged condition after 130 cycles for cells cycled at -10, 0, 25 and 40°C.
The XRD pattern of anode after 130 cycles in charged condition contained Pb (cubic) and PbSO4 (anglesite) peaks, like that for the fresh cells (Figure 9a). Along with Pb and PbSO4, there were PbO, Pb3O4 peaks for the cycled cell at different temperatures (Figures 9 a-d). Pb peaks intensity was higher and sharper in the new cell than the cycled cells, which confirmed that the PbSO4 was completely converting into Pb through reduction reaction. However, there was a permanent deposition of PbSO4 in the cycled cells after 130 cycles due to surface hardening. The intensity of PbSO4 peaks was more intense at 40 °C compared to 25 °C and new cell, which could be attributed to poor reversibility with a higher amount of PbSO4 at the anode in the charge condition (Figure 9a).
The cathode in charged condition contained PbO2 and PbSO4 peaks, like that of the fresh cells (Figure 9b). The cell after 130 cycles also showed Pb, PbO, and Pb3O4 peaks. The PbO2peaks intensity was higher and sharper in the fresh cell than the cycled cell after 130 cycles. PbSO4 detected due to permanent deposits (surface hardening) through charge/discharge cycling, leading to capacity loss. The PbSO4 peaks was more intense at 40 °C compared to those cycled at 25 °C and fresh cell due to accelerated kinetics at higher temperature leading to irreversibility. However, there was not much difference observed in the cells at 0 °C in charged (Figure 9a) than cell in discharged state (Figure 9c). This could be because at 0 °C only half of the cell capacity was used, discharge capacity is 4.1 Ah at 25 °C and 2.15 Ah at 0 °C due to the change in kinetics at 0 °C temperature. Similarly, at -10 °C the cathode for the cell after 130 cycles in charged and discharged condition contained PbO2, Pb3O4 and PbSO4 peaks (Figure 9b & d). However, at 0 °C, the anode both in charged and discharged conditions contained Pb (cubic), PbO and PbSO4 (anglesite) peaks (Figure 9a & c).
The anode in discharged condition for the cell after 130 cycles contained the Pb (cubic) and PbSO4 (anglesite) peaks, like that of the fresh cells (Figure 9c). Along with Pb and PbSO4, there were Pb3O2SO4, PbSO3, PbO, and Pb3O4peaks in the cell after 130 cycles. The PbO, PbSO4, and Pb3O4 peaks were identified in Figures 9c & 9d. \soutThe Pb3O2SO4 peaks were not detected in XRD, but they do exist in charging condition. The PbSO4 peaks intensity was higher and sharper in the fresh cell compared to the cell after 130 cycles, mostly agglomerated and surface hardened active materials, which confirmed that the Pb was completely converting into PbSO4 through an oxidation reaction in the fresh cell.6,17,18,23 Some of the PbSO4 peaks such as 26.6°, 29.8° and 36° are relatively higher in the cycled cell compared to the fresh cells. However, the main peak intensity at 44.4° is more intense in the fresh cell than cycled cells. This discrepancy observed could be attributed to the compositional difference on the electrode surface and in the bulk. The Pb peaks were more intense at 40 °C compared to that at 25 °C and fresh cell, this is probably due to higher deposition of Pb at anode in the charged condition.6,23
The cathode in discharged condition contained PbO2 and PbSO4 peaks for the cycled cells, like that with the fresh cells (Figure 9d). Moreover, the cell after 130 cycles also showed Pb3O2SO4, PbSO3, Pb, PbO, and Pb3O4 peaks. The PbSO4peaks intensity were higher and sharper in the new cell compared to the cell after 130 cycles. The PbO2 detected could be due to the inability of the bulk active material not being able to get reduced during discharge at elevated temperature, leading to less utilization of active materials and faster capacity fading/degradation in the cycled cell. Obviously, the PbO2 peaks was more intense at 40 than compared to 25 °C and the fresh cell due to larger deposition of PbO2 at the cathode in the charged condition.13,14 There was not much difference observed in the Pb peak intensity in charge condition for cathode (Figure 9b) and cell at discharge condition (Figure 9d) at 0 °C. Similar to anode, this could be because at 0˚C only half of the cell capacity was used due to the slower kinetics at this temperature. In summary, the XRD analysis concludes that the major/minor phases in charged and discharged state for electrodes are as illustrated in Table 1.
Table 1: Summary of phases present in the electrode from XRD analysis.