Zr-doped P2-Na0.75Mn0.55Ni0.25Co0.05Fe0.10Zr0.05O2 as high-rate performance cathode material for sodium ion batteries

In this work, Zr4+ doped P2-Na0.75Mn0.55Ni0.25Co0.05Fe0.10Zr0.05O2 has been synthesized and investigated as high-rate sodium-ion battery cathode material for the first time. XRD results indicate that Zr-ion with a big size cannot completely dissolve into the crystal structure, but the residual ZrO2 shows little impact on the electrochemical performance in comparison with the post-added ZrO2. With the substitution of Zr4+ for Fe3+, the lattice parameter a decreases but c increases, resulting in the reduced lattice volume. In comparison with the Zr-free sample, Na0.75Mn0.55Ni0.25Co0.05Fe0.10Zr0.05O2 (Zr05) can deliver a discharge capacity of about 95, 74, 61 and 53 mAh g−1 at 2C, 5C, 8C and 10C. At the current density of 2C, the capacity retention after 100 cycles has increased from 57% of Zr0 to 74% of Zr05. The results demonstrate that Zr-ion substitution can effectively improve the rate performance and cycling stability of P2-structure cathode materials. The role of Zr4+ can be summarized as follows: (1) Zr-O (△Hf298K(Zr-O) = 760 kJ mol−1) has a stronger bond energy than that of FeO (△Hf298K(Fe-O) = 409 kJ mol−1), which enhances the structure stability and cycling performance; (2) the aliovalent substitution of Zr4+ for Fe3+ enhances the electronic conductivity, which is favorable for the rate capability; (3) the substitution of Zr4+ for Fe3+ can be beneficial to Na+ conduction because it increases disorder in the transition metal (TM) layers and can prevent Na+/vacancy ordering, which further enhances the rate capability.