Direct Contact Presodiation of Na0.67Fe0.5Mn0.5O2 Forms Jahn–Teller Distorted P′2 NaFe0.5Mn0.5O2 Phase

Abstract The P2-type Na0.67Fe0.5Mn0.5O2 (NFMO) is an attractive positive electrode for Na-ion batteries. However, NFMO’s capacity is limited by the amount of sodium stored in its initial framework. Here, we show that direct contact presodiation is a facile way to introduce cyclable sodium into NFMO before cycling and increases first cycle desodiation to 241 mAh/g, exceeding its theoretical capacity. We characterize the structural and surface changes that occur during presodiation. We report full conversion to the Jahn–Teller distorted P′2 phase, NaFe0.5Mn0.5O2, before cycling using presodiation. Furthermore, a NaF, PF5–, and Na2CO3-rich interphase is created on PS-NFMO. With electrochemical impedance spectroscopy and distribution of relaxation times analysis, we show that the charge-transfer impedance contribution is reduced due to differences in interphase speciation of NFMO and PS-NFMO. Lastly, we show that a robust cathode electrolyte interphase is created during presodiation, which enables high coulombic efficiency (>90%).