Table I lists the electron (⇣ e ) and hole mobilities (⇣ h ), as well as the electronic conductivity ( e ) for all the four SEI components formed under di↵erent electrochemical conditions at T formation = 300 K (data for T formation = 500 K can be found in Table of SI). The electron mobilities are lower in oxides and phosphates compared to those in the nitride and phosphide. Combined with the low predicted free charge carrier concentrations, Li 2 O and Li 3 PO 4 show negligible electronic conductivity irrespective of the electrochemical conditions. On the other hand, Li 3 N and Li 3 P show very high e reaching 10 1 cm 3 and 10 4 cm 3 , respectively, at Li-rich conditions. The lower limits of e at the most Li-poor conditions of Li 3 N and Li 3 P are 10 13 cm 3 and 10 10 cm 3 , respectively, which is still non-negligible for an SEI. Note that the free carriers are always electron in the various materials and electrochemical conditions. Section II B shows a case of Li 2 O formed at 800 K (though not achievable in the SEI) where holes become the free carriers. e also always increases as the chemical condition becomes more Li-rich or the voltage vs. Li/Li + decreases. In summary, Li 3 N and Li 3 P show worrying electronic conductivity that enable the Li-LiPON interfacial reactions (decomposition of LiPON) and the concomitant SEI formation.