Unnormalized operando spectra, obtained throughout a CV of HC-PB, are shown in Figure , alongside the determined compositions of potassium within PB, PW, and electrolyte environments (Figure ). The compositions are determined from the potassium K-edge step values and associated fractions of each K + species determined by LCF analysis (Figure ). Visual inspection of the unnormalized spectra in Figure shows a clear evolution of speciation through the voltage sweep, and a change in edge step height. As K + speciation evolves through the different environments of PB and residual electrolyte, the edge step decreases as K + is removed from the cathode. The edge step then increases as the sample is oxidized to PB and then PW. Consequently, comprehensive analysis of both K + speciation and its relative concentration within the PB cathode, with respect to applied potential and current response, can be made during charge/discharge cycling.