Achieving Long-Enduring High-Voltage Oxygen Redox in P2-Structured Layered Oxide Cathodes by Eliminating Nonlattice Oxygen Redox.

ABSTRACT

Triggering reversible lattice oxygen redox (LOR) in oxide cathodes is a paradigmatic approach to overcome the capacity ceiling determined by orthodox transition-metal (TM) redox. However, the LOR reactions in P2-structured Na-layered oxides are commonly accompanied by irreversible nonlattice oxygen redox (non-LOR) and large local structural rearrangements, bringing about capacity/voltage fading and constantly evolving charge/discharge voltage curves. Herein, a novel Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 (◻ = TM vacancies) cathode with both NaOMg and NaO◻ local configurations is deliberately designed. Intriguingly, the activating of oxygen redox at middle-voltage region (2.5-4.1 V) via NaO◻ configuration helps in maintaining the high-voltage plateau from LOR (≈4.38 V) and stable charge/discharge voltage curves even after 100 cycles. Hard X-ray absorption spectroscopy (hXAS), solid-state NMR, and electron paramagnetic resonance studies demonstrate that both the involvement of non-LOR at high-voltage and the structural distortions originating from Jahn-Teller distorted Mn3+ O6 at low-voltage are effectively restrained in Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 . Resultantly, the P2 phase is well retained in a wide electrochemical window of 1.5-4.5 V (vs Na+ /Na), resulting in an extraordinary capacity retention of 95.2% after 100 cycles. This work defines an effective approach to upgrade the lifespan of Na-ion battery with reversible high-voltage capacity provided by LOR.