New Insights into Anionic Redox in P2-Type Oxide Cathodes for Sodium-Ion Batteries.

Manganese/nickel-based layered transition metal oxides have caught the attention of studies as promising cathodes for sodium-ion batteries (SIBs). It is reported that utilizing both cationic and anionic redox reactions is a promising method for higher energy density cathodes. However, the anionic redox reaction comes at the expense of irreversible oxygen release. Hence, a Li-Mg cosubstituted P2-Na _0.67 Li _0.07 Mg _0.07 Ni _0.28 Mn _0.58 O ₂ material with a honeycomb-ordered superstructure was designed. The Ni ³⁺ /Ni ⁴⁺ redox couple and the anionic redox reaction are proven to have a competitive relationship. Density functional theory calculations reveal the effect of O 2p nonbonding states from Li and prove that Mg-O bonds can stabilize the Ni-O e _g states. In situ electrochemical impedance spectroscopy measurements and galvanostatic charging/discharging derived d V /d Q , representing resistance changes with time, are obtained to reveal the mechanism of the anionic redox reaction. This study presents the effect and mechanism of the O 2p nonbonding state and Mg-O bonds of manganese/nickel-based layered oxides.