Under the most Li-poor (oxidizing) conditions (Figure ), E Fermi,eq is 1.85 eV. The dominant defects are V - 1 Li , Li +1 i , and O +1 i , and their E D,q are very close to each other (1.21 eV). The Li-related dominant defects show a total concentration of 6.11 ⇥ 10 2 cm 3 . The defects with positive and negative charges show almost identical concentrations, and the net free carrier concentration is ⇠0.0 cm 3 . Note that the carrier (hole) con-centration can reach 2.80 ⇥ 10 9 cm 3 at a usual (though not achievable in the Li-LiPON SEI formation) synthesis temperature of 800 K. From these results, it is obvious that the SEI component Li 2 O is highly insulating in the accessible range of thermodynamic equilibrium growth conditions and relevant formation temperatures.