Enhancing the Rate Capability and Cycling Stability of Na0.67Mn0.7Fe0.2Co0.1O2 through a Synergy of Zr4+ Doping and ZrO2 Coating

Sodium-ion batteries (SIBs) have aroused great interest as large-scale energy storage devices because of the abundant Na resource. However, the lack of high-performance cathode materials is still a big challenge for the practical application of SIBs. Herein, the synergic modification of Zr4+ doping and ZrO2 coating on P2-structure Na0.67Mn0.7Fe0.2Co0.1O2 (MFC) has been achieved by a facile Zr(OC4H9)4-mediated sol–gel method. The rate capability and cycling stability are simultaneously enhanced, and their synergetic mechanism is revealed. The enhancement of the rate capability is largely attributed to the expansion of the interlayer spacing and the enlargement of Na–O bond length, which decreases the Na+ migration barrier and the electrostatic attraction between Na and O. This facilitates Na ions intercalation/extraction and enhances the rate capability. The improvement of the cycling stability is first attributed to the protection of ZrO2 coating, which reduces the side reactions between the electrode and electrolyte and benefits to the stability of the layered structure. In addition, doping of Zr4+ also reduces the bond length of TM–O/O–O and increases its bonding energy, which further enhances the layered structure stability. Last but not least, the relative content of Mn3+ is also mitigated which alleviates Jahn–Teller distortion and further enhances the structure stability. In situ X-ray diffraction is also performed to probe the structure evolution of ZrO2@MFC during the sodiation/desodiation. The proposed synergetic strategy is also suitable to modify other cathode materials.