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Elements doping has been used to improve the electrochemical performances of O3-type layered transition metal oxide cathodes for sodium-ion batteries. However, their roles and the improvement mechanism have not been clearly understood. Herein, the effects of Mg substitution for Mn on the structure and electrochemical performances of NaMnNiFeMgO have been comprehensively investigated and some new insights into the roles of Mg in improving the rate capability and cycling stability have been presented. (1) The substitution of Mg for Mn enlarges the interlayer spacing, which not only enhances Na diffusion and the rate capability but also alleviates the lattice strains induced by Na intercalation/deintercalation. (2) The substitution of Mg by Mn also shrinks TM-O bond and TMO slabs, which enhances the layered structure stability. (3) The Mg substitution also mitigates the structure distortion or volume change of the crystal lattices and suppresses the irreversible phase transitions. (4) The substitution of low-valence Mg for Mn reduces Mn and minimizes Jahn-Teller effect, which also further alleviates the irreversible phase transformations and improves the layered structure stability. This study not only unveils the roles of Mg but also presents some insights into designing the cathode materials with both high rate capability and high cycling stability through the lattice structure regulation.

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