## 3 Results
### Operando XANES and MCR-ALS Data Analysis  
The operando dataset recorded during charge (from spectrum 4 \(\equiv\) D\({}_{0}\) to spectrum 64 \(\equiv\) C\({}_{1}\)) and discharge (from spectrum 65 to spectrum 123 \(\equiv\) D\({}_{1}\)) at the Mn and Fe K-edges is presented in Figure 4. The results point out: (1) the high variability of the Mn K-edge and low modification at the Fe K-edge; (2) the energy shift occurring at both Mn and Fe sites during charge (shift towards higher energies up to C\({}_{1}\) state of charge) and discharge (shift towards lower energies up to D\({}_{1}\) state of charge). The large dataset acquired at both edges was analyzed with the MCR-ALS chemometric approach, revealing the three pure spectral components necessary to describe the electrochemical cycling processes in the present case study and the concentration profile plot for each component.  
Figure 3: Spectra at Mn and Fe K-edges for the initial, charged, and discharged states of charge. Each panel illustrates the comparison of the spectra recorded in the previous work and the ones reported in Reference [11]. The Mn K-edge for the initial, charged, and discharged states is displayed in panels (**a**), (**c**), and (**e**), respectively, while the analog series at the Fe K-edge is displayed in panels (**b**,**d**,**f**).  
According to the PCA analysis (cf. SI), a minimum of three different pure spectral components is necessary to interpret the complete set of data at both edges. The unique components describing the whole cycling process are presented in Figure 5a,b for Mn and Fe, respectively, while the corresponding concentration profile plots are reported in Figure 5c,d. Contrary to what has been already reported for the same system under the conventional charge/discharge cycling protocol [12], the initial state Mn (D\({}_{0}\)) is not uniquely described by a single component, i.e., \"Component 1\", but also by an additional contribution, labeled \"Component 2\", which accounts for 10% in D\({}_{0}\) (cf. Panels a,c). Due to the mixed character observed for D\({}_{0}\), the respective spectrum does not match the pref spectrum in Figure 3a. The electrochemical protocol here adopted induces the formation of a new spectral component (Component 2) already in D\({}_{0}\), which, during charge, reaches a maximum in concentration around spectrum 35 for both sets of data (cf. Panels c,d). Despite the presence of mixed contributions in the initial state and during charge, Component 3 completely represents the fully charged state (C\({}_{1}\)) at both edges, in line with the comparison and good agreement with Cref spectra in Figure 3c,d. During the following discharge, pure spectral Component 1 and Component 2 rise at the expense of the third one, which tends to disappear in the fully discharged state. Component 2 is present in a relatively wide range of spectra (i.e., states of charge), largely persisting in D\({}_{1}\). On the other hand, the Fe in D\({}_{1}\) almost entirely consists of Component 1 as the initial state, i.e., Fe D\({}_{0}\)-Fe D\({}_{1}\). The discrepancy between Mn D\({}_{0}\) and Mn D\({}_{1}\) states, already visible from Figure 2, is confirmed by the MCR-ALS analysis and attributed to the anomalous charge/discharge protocol to which the cell was subjected.  
Figure 4: X-ray absorption fine structure (XAFS) operando dataset at the Mn and Fe K-edges and the respective electrochemical curve.  
Figure 5: Multivariate Curve Resolution—Alternating Least Square (MCR-ALS) retrieved pure spectral components at the (**a**) Mn and (**b**) Fe K-edges and respective concentration profile plots (**c,d**).