Identifying Anionic Redox Activity within the Related O3- and P2-Type Cathodes for Sodium-Ion Battery

As promising cathodes for Na-ion batteries (NIBs), layered transition-metal (TM) oxides have attracted intense research activities because of high specific capacities, especially benefiting from the boosted capacity triggered by oxygen-related anionic redox reactions (ARRs). However, regarding ARRs activity, the difference between typical O3- and P2-type structures has not been clarified with in-depth exploration. Herein, composed with similar composition, ARRs-induced oxygen behaviors within O3-Na_0.6Li_0.2Fe_0.4Ru_0.4O₂ and P2-Na_0.6Li_0.35Fe_0.1Ru_0.55O₂ are systematically investigated by varying ex/in situ spectroscopic characterizations. Conducted with a lower charging cutoff voltage (4.0 V), P2-type cathode will more easily trigger the reversible oxygen behaviors and deliver a larger capacity, better rate performance, and stable cyclability, in contrast to the O3-type cathode. Moreover, within O3-type structure, increasing charging potential (beyond 4.3 V) would induce additional anionic oxidation capacity, but inevitably lead to the irreversible evolution of gaseous O₂ and superoxo. With the unique feature, this work provides a promising strategy design for fabricating cathodes with optimal microstructural arrangement, which could further push forward the changes in macro-/nanostructures and even ideal performance.