Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries.

Though considerable Mg-doped layered cathodes have been exploited, some new differences relative to previous reports can be concluded by doping a heavy dose of Mg via two rational strategies. Unlike the common unit cell of the P63/mmc group by X-ray diffraction, neutron diffraction reveals a large supercell of the P63 group and enhanced ordering for Na11/18Mg1/18[Ni1/4Mg1/9Mn11/18]O2 with Mg occupying both the Na and Mn sites. Compared with only one obvious voltage plateau of Na0.5[Ni0.25Mn0.75]O2 (NNM), Na11/18Mg1/18[Ni1/4Mg1/9Mn11/18]O2 (NMNMM) shows more severe voltage plateaus but with excellent electrochemical performance. Na0.5[Mg0.25Mn0.75]O2 (NMM) with Mg only occupying the Ni site displays a highly reversible whole-voltage-range oxygen redox chemistry and smooth voltage curves without any voltage hysteresis. Cationic Ni2+/Ni4+ couples are responsible for the charge compensations of NNM and NMNMM, while only the oxygen anionic reaction accounts for the capacity of NMM between 2.5 and 4.3 V. Interestingly, the Mn3+/Mn4+ pair contributes all capacity for all cathodes between 1.5 and 2.5 V. All cathodes undergo a double-phase mechanism: an irreversible P2-O2 phase transition for NNM, an enhanced reversible P2-O2 phase transition for NMNMM, and a highly reversible P2-OP4 phase transition for NMM. In addition, the designed cathodes display excellent rate capability and long-term cycling stability but with a large difference in the various voltage ranges of 2.5-4.3 and 1.5-2.5 V, respectively. This work provides a good understanding of ion doping and some new insights into exploiting high-performance materials.