To illustrate the significance of observed grey-level changes, a Z-test was performed using data from Case Study 2. The null hypothesis (H 0 ) assumed no change in the population mean grey-level between the pristine state and 4.0 V SoC (i.e. any observed change is a random fluke of sampling), while the alternative hypothesis (H A ) posited a decrease in mean grey-level at 4.0 V. The Z-statistic for this hypothesis test was calculated as: The critical value for a one-tailed test with a 5σ significance level is V crit = -5.01. Since the calculated Z-statistic (Z = -5.40) is less than the critical value, the null hypothesis is rejected. This confirms that the observed decrease in grey-level is statistically significant and not to random sampling.