According to Eq. 3, e is a function of the free charge carrier concentration (n), which is determined from the defect calculations. Because we consider charged defects as the primary source of free charge carriers, n depends on the synthesis/formation temperature and the prevailing electrochemical conditions. In addition to the charged point defects, polarons may form especially in polarizable materials like Li 2 O. If polaronic e↵ects are taken into account for Li 2 O, our calculated n will be lowered further because the charge carriers will be trapped as polarons. Consequently, e will be reduced even further. As discussed later, this does not alter our conclusion that Li 2 O is an electronic insulator.