Achieving complete solid-solution reaction in layered cathodes with reversible oxygen redox for high-stable sodium-ion batteries

Abstract

P2-type layered Mn-based oxides are promising cathode materials for sodium-ion batteries (SIBs), but it is still challenging to achieve both high capacity and stability because of complex phase transitions and irreversible oxygen release at high voltage. To address these challenges, an optimal P2-type Na_0.67Mn_0.8Cu_0.15Ti_0.05O₂ (NMCT) cathode with a complete solid-solution reaction and reversible oxygen redox reaction over a wide voltage range was developed. The introduction of the Na–O–Ti configuration leads to fewer delocalized electrons on oxygen and thus enhances oxygen redox activity, while the high energetic overlap between O 2p and Cu 3d states and the increased Mn–O hybridization strengthen the rigidity of oxygen framework to achieve reversible and stable oxygen redox reaction. In addition, the reinforced TM–O interaction, combined with the ameliorated Mn³⁺ Jahn-Teller distortion and disrupted Na⁺/vacancy ordering, synergistically eliminate the undesired P2–OP4 phase transition and lead to a complete solid-solution reaction, which greatly facilitates Na⁺ transport kinetics and stabilizes structural integrity. As a consequence, improved rate performance and cycling stability are achieved for NMCT. Our present study provides a promising avenue for simultaneously utilizing the reversible oxygen redox activity and maintaining the structural integrity to accomplish the capacity-stability trade-off of Mn-based oxide cathodes for constructing practical SIBs.