Figure 4: Charge curves at C/10 rate for
(a ) -10 °C, (b) 0 °C, (c)
25 °C and (d) 40 °C
Discharge capacity of the cells calculated from the discharge profiles
is illustrated in Figure 5a. It was observed that discharge capacity is
close to the theoretical capacity (5Ah) for 25 °C and 40 °C, while the
initial discharge capacity at -10 °C is higher than the result at 0°C,
the discharge capacity drop is much significant in -10 °C because lower
temperature increases the internal resistance and reduces the
capacity.19 Steep slope confirms that degradation is
higher at 40 °C due to enhanced ageing of the electrodes from faster
reaction kinetics at elevated temperature. The capacity degradation
after 130 cycles are 92, 74, 69 and 17% for 40, -10, 25 and 0 °C,
respectively. Moreover, the cell degradation is insignificant at 0 °C
even though the total discharge capacity is low. The state of health
(SOH) is a measurement that reflects the general condition and ability
to deliver the specified performance compared to that of a fresh
battery. SOH takes into consideration factors such as charge acceptance,
internal resistance, voltage, and
self-discharge.4,20,21 It is a measure of the
battery’s long-term capability and available lifetime energy throughput.
Health of the battery tends to deteriorate gradually due to irreversible
physical and chemical changes, hence SOH is estimated by multiple
methods including impedance, conductance and discharge capacity
(current/actual).4,20 SOH was calculated using instant
discharge capacity and initial capacity (after 1stcycle) as illustrated in Figure 5b. SOH was observed to be highest for 0
°C ranging from 95% (1st cycle) to 89%
(130th cycle). However, the SOH decreased sharply for
cells discharged at -10 °C (78% to 46%), 25 °C (67 to 28%), and 40 °C
(73 to 10%). Significant SOH loss was observed for 40 °C confirming
major degradation.