# Li2O-Based Cathode Additives Enabling Prelithiation of Si Anodes
## 3 Results and Discussion
### Performance of Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm Electrode  
Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) was planetary ball-milled at 1100 rpm for 5 min with a 25 min rest, and this sequence of steps was repeated four times. When added to the NMC622 electrode, these additives successfully delivered additional capacity at a voltage below 4.0 V in the half cells (Figure 2a). Note that the Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @400 rpm electrode showed additional capacity starting at -4.35 V vs. Li/Li\({}^{+}\) (Figure 1b), demonstrating our approach of milling at higher revolutions per minute led to the enhanced performance of these additives. The charging profile of Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm exhibited a slope change at -4.0 V, which appears as a distinct peak in the dQ/dV plot (Figure 2a inset). This feature corresponds to the plateau observed from the charging profiles of Li-O\({}_{2}\) batteries with Co\({}_{3}\)O\({}_{4}\) catalyst [30; 31]. The additional capacity obtained from Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm by charging up to 4.3 V vs. Li/Li\({}^{+}\) was 1116 mAh/g\({}_{\text{LI2O}}\), excluding the mass of Co\({}_{3}\)O\({}_{4}\). Considering the total mass of both Li\({}_{2}\)O and Co\({}_{3}\)O\({}_{4}\), the additional capacity was 589 mAh/g\({}_{\text{LI2O}}\)+Co\({}_{3}\)O\({}_{4}\). The low activation voltage and considerable capacity obtained below 4.3 V vs. Li/Li\({}^{+}\) from this Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrode is promising, as the higher voltage required for complete activation of Li\({}_{2}\)O could be detrimental to cell performance by overcharging the cathode and damaging the CEI [32; 33]. When the electrode was charged up to 4.7 V vs. Li/Li\({}^{+}\), the reaction of Li\({}_{2}\)O continued and an additional capacity of 1504 mAh/g\({}_{\text{LI2O}}\) was obtained, which corresponds to 793 mAh/g\({}_{\text{LI2O}}\)+Co\({}_{3}\)O4 (Figure S3). While the higher capacity per gram of Li\({}_{2}\)O from the  
Figure 2: (a) Half-cell and (b) full-cell profiles of the baseline and the additive Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm containing electrodes during the first cycle. Half cells and full cells were charged at a C/20 rate up to 4.3 and 4.6 V, respectively. Voltage profiles are plotted as a function of grams of NMC in the electrode, excluding the mass of additives. dQ/dV plots for half cells are shown as an inset of (a). Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrode compared to that from the Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @400 rpm electrode (1504 vs. 1163 mAh/g\({}_{\text{Li2O}}\) with 4.7 V charging) can be attributed to the smaller particle size, as discussed in Figure 1b, the significantly lower overpotential is surprising as there was no modification of the Co\({}_{3}\)O\({}_{4}\). To further understand the source of this behavior, we performed X-ray diffraction (XRD) and XPS analyses of the Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @400 rpm and Li\({}_{2}\)O + Co\({}_{3}\)O\({}_{4}\) @1100 rpm electrodes. From the XRD spectra (Figure S4), we were not able to detect a signal from the additives likely due to their relatively low content. While the XPS spectra (Figure S5) captured the transition metal core levels (Ni 2p, Co 2p, and Mn 2p), no noticeable difference was observed, even from the Co 2p signals. Thus, the lower overpotential likely resulted from morphological rather than chemical factors. Previously, [PERSON] et al. performed transmission electron microscopy (TEM) analysis on their Co/Li\({}_{2}\)O nanocomposite and showed that having smaller, uniformly distributed Co in intimate contact with Li\({}_{2}\)O is key to enhancing the additive performance [23]. Thus, milling at higher revolutions per minute not only makes the particle size smaller, but can also enhance the contact between Li\({}_{2}\)O and Co\({}_{3}\)O\({}_{4}\), leading to the lower over-potential.