# Li2O-Based Cathode Additives Enabling Prelithiation of Si Anodes
## 3 Results and Discussion
### Analysis of the Cathode Surface Chemistry  
The XPS spectra of baseline and Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrodes cycled between 3.0 and 4.7 V in half cells are shown in Figure 3. The electrodes were analyzed at their discharged states. C 1s, F 1s, and P 2p core levels showed peaks associated with electrolyte decomposition products. While C 1s core-level peaks (carbon black at 284.5 eV, C-C/C-H at 284.8 eV, C-O at 286.7 eV, O-C = O at 289.0 eV, and CO\({}_{3}\) at 291.2 eV) [34; 35; 36; 37] showed similar intensities, the F 1s and P 2p core-level signals were stronger from the additive-containing electrodes. For example, the Li\({}_{x}\)PF\({}_{y}\) species (F 1s peak at 689.0 eV and P 2p peak at 138.4 eV) [34; 37], which is absent in the baseline electrode, was observed from the Li\({}_{2}\)OThese observations indicated the extensive salt decomposition taking place in the presence of additives. The formation of fluorophosphates is initiated by the hydrolysis of LiPF\({}_{6}\) and the reaction continues as the fluorophosphate species further react with additional water or solvents to form phosphates [38; 39]. These reactions are accelerated by the presence of reactive oxygen species, such as peroxides and superoxides, created as intermediates during the charging of Li\({}_{2}\)O [25; 26; 40]. In addition, the amount of phosphates present on the electrode is proportional to the degree of capacity fade observed after the first charge. Comparing XPS spectra readings of the Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrode charged up to 4.3 and 4.7 V (Figure S9), a stronger phosphate signal was observed from the 4.7 V charged electrode, which showed greater capacity fade (i.e., 18 vs. 7 mAh/gNMC).  
Finally, looking at the transition-metal core levels, the Ni 2p and Mn 2p spectra showed similar signals from both the baseline and Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrodes. On the other hand, the Co 2p spectra showed a distinctly stronger signal from the additive-containing electrode, indicating the presence of Co\({}_{3}\)O\({}_{4}\)[41] after the activation of Li\({}_{2}\)O. Interestingly, prior to cycling, the Co 2p signal from the Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrode was similar to that of the baseline electrode (Figure S5). Thus, Co\({}_{3}\)O\({}_{4}\) may be migrating closer to the surface as Li\({}_{2}\)O reacts and becomes converted to O\({}_{2}\). While it may seem that the Co\({}_{3}\)O\({}_{4}\) concentrating at the surface could be impacting the utilization of NMC in the following cycles, we note that capacity fade was also observed from Li\({}_{2}\)O-only electrodes. For  
Figure 3: XPS spectra of (**a**) baseline and (**b**) Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrodes cycled between 3.0 and 4.7 V. C 1s, F 1s, P 2p, Ni 2p, Mn 2p, and Co 2p core levels are shown.  
example, the discharge capacity of the Li\({}_{2}\)O @400 rpm electrode was 196 mAh/g\({}_{\text{SMC}}\) while that of the baseline electrode was 229 mAh/g\({}_{\text{SMMC}}\), exhibiting a 33 mAh/g\({}_{\text{SMMC}}\) capacity fade (Figure S8b). Therefore, electrolyte decomposition and the creation of (fluoro)phosphate-rich CEI are the major factors damaging electrode performance.