As an example, Li 3 P is in equilibrium with elemental Li and LiP. It is evident that Li metal represents the most Li-rich limit (most reducing condition) and LiP the most Li-poor (most oxidizing) limit for Li chemical potential (µ Li ). A more Li-rich chemical condition corresponds to a more reducing electrochemical situation in batteries, or a lower voltage vs. Li/Li + . One can also find that there is only one independent variable of elemental chemical potential in the binary Li 2 O is a highly stable oxide and a common SEI component at the Li anode when the electrolyte contains oxygen. We calculated the formation energy of native defects in Li 2 O as a function of the Fermi energy (Figure ). The Fermi energy (E Fermi ) is referenced to the valence band maximum (VBM) i.e., E Fermi = 0.0 eV at VBM. The highly ionic Li 2 O has a ultra-wide band gap of ⇠8.01 eV (with GW approximation), which sets the conduction band minimum (CBM) to E Fermi = 8.01 eV. It is notable that the GW approximation tends to slightly overestimate experimental band gaps of ultrawide band gap battery materials. We consider the two limiting thermodynamic conditions within the region of phase stability: (i) Most Li-rich condition (reducing) corresponding to equilibrium with metallic Li ( µ Li = 0 eV, voltage vs. Li/Li + is 0 V), and (ii) Most Li-poor condition (oxidizing) corresponding to equilibrium with Li 2 O 2 ( µ Li = 3.02 eV, voltage vs. Li/Li + is 3.02 V). Figures 3(a) and 3(b) show the defect formation energies under the most Li-rich and most Li-poor conditions, respectively. In Figure and subsequent defect plots, all unique Wycko↵ sites for each defect type are considered and plotted. In Li 2 O, there is only one unique Wycko↵ site each for Li and O. The slopes of the defect lines are the charge states of the defects, as evident from Eq. 2.