As a potential Li-ion solid conductor, the defect thermodynamics of Li 3 P has been the subject of previous studies. The high defect concentration in Li 3 P was found to provide ionic conductivity higher than 10 4 S/cm. Our GW calculated band gap of Li 3 P is 2.14 eV, which is similar to the band gap of Li As in Li 3 N, E Fermi,eq for Li 3 P under the Li-rich condition (Figure ) is inside the conduction band (2.55 eV), and yields degenerate free electron concentration of 5.66 ⇥ 10 20 cm 3 . The dominant defects are V +1 P , Li +1 i , and V - 1 Li , and the total concentration of defects that mediate Li-ion migration is 8.11 ⇥ 10 12 cm 3 . E Fermi, eq moves to a lower energy (1.87 eV) under the moderate condition (Figure ). Under the moderate condition, the dominant defects are Li +1 i and V -1