Enabling Anionic Redox Stability of P2-Na_5/6 Li_1/4 Mn_3/4 O₂ by Mg Substitution

Oxygen-based anionic redox reactions have recently emerged as a lever to increase the capacity of Mn-rich layered oxide cathodes in addition to the charge compensation based on cationic redox reactions for sodium-ion batteries. Unfortunately, the irreversibility of anionic redox often aggravates irreversible structure change and poor cycling performance. Here, a stable anionic redox is achieved through substituting Na ions by Mg ions in P2-type Na_0.83 Li_0.25 Mn_0.75 O₂ . Density functional theory (DFT) calculations reveal that Mg substitution effectively decreases the oxygen chemical potential, causing an improvement in lattice oxygen stability. Moreover, at a highly desodiated state, Mg ions that remain in the lattice and interact with O 2p orbitals can decrease the undercoordinated oxygen and the nonbonded, electron-deficient O 2p states, facilitating the reversibility of oxygen redox. When cycled in the voltage range of 2.6-4.5 V where only anionic redox occurs for charge compensation, Na_0.773 Mg_0.03 Li_0.25 Mn_0.75 O₂ presents a much better reversibility, giving a 4 times better cycle stability than that of Na_0.83 Li_0.25 Mn_0.75 O₂ . Experimentally, Na_0.773 Mg_0.03 Li_0.25 Mn_0.75 O₂ exhibits a ≈1.1% volume expansion during sodium insertion/extraction, suggestive of a "zero-strain" cathode. Overall, the work opens a new avenue for enhancing anionic reversibility of oxygen-related Mn-rich cathodes.