New Insights\ninto Anionic Redox in P2-Type Oxide Cathodes\nfor Sodium-Ion Batteries

Manganese/nickel-based layered transition metal oxides\nhave caught\nthe attention of studies as promising cathodes for sodium-ion batteries\n(SIBs). It is reported that utilizing both cationic and anionic redox\nreactions is a promising method for higher energy density cathodes.\nHowever, the anionic redox reaction comes at the expense of irreversible\noxygen release. Hence, a Li–Mg cosubstituted P2-Na<sub>0.67</sub>Li<sub>0.07</sub>Mg<sub>0.07</sub>Ni<sub>0.28</sub>Mn<sub>0.58</sub>O<sub>2</sub> material with a honeycomb-ordered superstructure was\ndesigned. The Ni<sup>3+</sup>/Ni<sup>4+</sup> redox couple and the\nanionic redox reaction are proven to have a competitive relationship.\nDensity functional theory calculations reveal the effect of O 2p nonbonding\nstates from Li and prove that Mg–O bonds can stabilize the\nNi–O e<sub>g</sub> states. <i>In situ</i> electrochemical\nimpedance spectroscopy measurements and galvanostatic charging/discharging\nderived d<i>V</i>/d<i>Q</i>, representing resistance\nchanges with time, are obtained to reveal the mechanism of the anionic\nredox reaction. This study presents the effect and mechanism of the\nO 2p nonbonding state and Mg–O bonds of manganese/nickel-based\nlayered oxides.