## 3 Results  
Spectra integrated over the whole field of view for the set of samples are reported in Figure 2. To only consider reliable absorbance we selected valid pixels with absorbance at the measured maximum B between 0.15 and 1.08, i.e., from image regions with sample and with an expected error below 10%; therefore, we consider their shape highly accurate. A measurement without spatial resolution would require a thin and highly homogeneous sample to warrant the same quality. The prepeaks in O-K spectra include transitions to empty oxygen 2p states that in the case of bonding with a TM are hybridized with its d-states. These peaks result as the most intense and sharp feature, and as shown previously their envelope reproduces closely the shape of Mn 3d states, which indeed is the most abundant TM in the active material [17]. To describe their evolution along the first charge-discharge we identified three main components: \(A\), \(B\), and \(C\), centered at 528.1 eV, 529.1 eV, and 530.3 eV, respectively. \(A\) is attributed to Ni(IV), formed upon initial Li deintercalation, before P03. \(B\) corresponds to the main Mn(IV) peak, while \(C\) emerges during the oxidation plateau at 4.5 V and has been attributed to both the formation of Mn(III), as evidenced by Mn-K edge XAS [17] and trapped molecular O\({}_{2}\), as recently discovered [14]. Intensities of molecular oxygen mainly overlap with other features, but the observed evolution between P03 and P05 is compatible with its presence. In fact, if corrected to our energy scale, peak \(C\) overlaps perfectly with transitions to O\({}_{2}\)\(\pi^{\bullet}\) states, and the increased intensity around 540 eV corresponds well to O\({}_{2}\)\(\sigma^{\bullet}\) reported in literature [21]. We suggested that Mn(III) is stabilized by the interaction with molecular O\({}_{2}\)[18], so that the two components are spatially highly associated in feature \(C\) within our resolution. The coating introduces a remarkable delay in the spectra evolution by changing, particularly in the Ni oxidation at P03 and in the growth of peak \(C\) at P04. An example of the transmission images relative to the spectra is reported in Figure 3 for the pristine VOx-coated sample. Figure 3a,b represent respectively the absorbance averaged in the flat O-K pre-edge region (i.e., immediately before the absorption prepeaks) and in the \(C\) region, where an absorption jump higher than one is present only in a minor fraction of the sample area. All components absorb similarly before the edge, but at the O prepeaks energy, the absorbance is much stronger only for the active material, which contains oxygen-transition metal bonds. The point-by-point difference is more obvious in Figure 3c, where these two maps are overlapped in a dual channel image, with red (pre-edge) and yellow (prepeak) colors. The red color prevails where only super C carbon black is present, in a finer texture, while most of the area has different orange fatalities that can be attributed to carbon mixed with coarser oxide particles. Figure 3d is the combination of the oxide (Figure 3b, yellow) with the V signal (green) of the coating. Indeed, smaller and isolated particles show more vivid green tonalities than larger aggregates where (surface) vanadium contribution is smaller. The fine red colored particles do not contain vanadium and are more difficult to recognize in Figure 3d. A full-field transmission spectromicroscopy measurement consists in acquiring several transmission images varying the energy across the absorption edge of interest, the oxygen K edge in the present case. After normalization, alignment, and conversion to absorbance, a full X-ray absorption spectrum (XAS) can be extracted at each image pixel, usually resulting in \(\sim\)\(1\times 10^{6}\) single pixel XAS (a typical field of view contains 1024 \(\times\) 1024 pixels). This large number of spectra complicates the extraction of scientifically meaningful information and efficient and reproducible data processing requires some level of automatization.