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-Na0.6Li0.2Fe0.4Ru0.4O2 and P2-Na0.6Li0.35Fe0.1Ru0.55O2 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.