Fast Li+ migration in LiPON electrolytes doped by multi-valent Fe ions

Achieving superior ionic conductivity of LiPON solid electrolyte films is critical for the solid-state thin-film batteries with high energy density. Here we describe a method of preparing LiPON with promising ionic migration capability and high work function by systematically tailoring the concentration of Fe ions doping. Fe-doped LiPON exhibits excellent ionic conductivity (1.08 × 10−5 S cm−1, nearly 10 times higher than the pristine LiPON), low ionic activation energy, and moderate equilibrium potential difference (versus LiCoO2, 0.78 V) at room temperature. The favorable ionic mobility and electrochemical stability of Fe-doped LiPON are fully confirmed. All-solid-state "Li/LiPON/LiCoO2" TFB has been successfully constructed with a large specific capacity (∼36.3 µAh cm−2 µm−1 at 10 µA cm−2) and good cycle performance (87.8 % capacity retention after 40 cycles). Fe with the unique d-orbital electronic structure changes the local electron density of LiPON system with the weakened electrostatic constraint of PO3N4− tetrahedrons to Li+. A low Li+ migration barrier center is established around the Fe–N bridge bonds.