Restraining Oxygen Loss and Boosting Reversible Oxygen Redox in a P2-Type Oxide Cathode by Trace Anion Substitution.

Oxygen redox has recently emerged as a lever to boost the specific energy density of layered sodium transition metal oxide cathode materials. However, the oxygen redox reaction is universally confronted with concomitant issues such as irreversible lattice oxygen loss and parasitical electrolyte degradation, thus debilitating cycling stability. Herein, a novel F-substituted layered structure P2-NaLiMnOF cathode is designed, which exhibits superb capacity retention (183.6 mAh g after 50 cycles at 0.05C, 87.8% of the highest discharge capacity) and rate capability (105.5 mAh g at 5C) in Na half-cells. Such results are nontrivial as this system only contains the low-cost Mn transition metal element. Moreover, by systematic bulk/surface spectroscopy evidence (hard and soft X-ray absorption spectroscopy, electron paramagnetic resonance, and operando differential electrochemical mass spectrometry), we explicitly corroborate that the irreversible oxygen evolution and notorious Jahn-Teller distortion are effectively subdued by trace F-substitution. In addition, a higher oxygen vacancy formation energy for the F-substituted structure was demonstrated via density functional theory calculations. Anionic substitution could therefore be an impactful solution to boost reversible oxygen redox chemistry for layered sodium oxide cathodes.