Synergistic effect of different configurations on the anionic redox reaction in Na-deficient oxides for sodium ion batteries

Utilizing the anionic redox reaction in P2 layered compounds is one way to improve the specific capacity of cathodes for sodium-ion batteries. In this work, we compare the difference and synergistic effects of various configurations including Na-O-Mg, Na-O-Zn and Na-O-Cu on the anionic redox reaction through systematic studies of Na0.67Mn0.7M0.3O2 and Na0.67Mn0.7M10.2M20.1O2 (M, M1, M2 = Mg, Cu, Zn). The work demonstrates that the reversibility of the anionic redox reaction and the structural stability of the oxides can be tailored by modulating different configurations. In (ex)-situ X-ray diffraction and the phase stabilities by DFT calculations indicate that with the increased bond strength from Mg-O, Zn-O to Cu–O bond in the TM layers, the structural phase transition from P2 lattice to O2 lattice in the compounds is much suppressed. In these compounds, the cathode material Na0.67Mn0.7Cu0.2Zn0.1O2 combining Na-O-Zn and Na-O-Cu configurations balances the anionic redox activity and the structural stability. Our experiments and calculations reveal that the relatively strong Zn-O and Cu–O interactions in Na0.67Mn0.7Zn0.1Cu0.2O2 account for the structural stability and improved electrochemical properties. The findings gained here may inspire the design and development of cathodes employing anionic redox chemistry for sodium-ion batteries.