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

In this work, Zr 4+ doped P2-Na 0.75 Mn 0.55 Ni 0.25 Co 0.05 Fe 0.10 Zr 0.05 O 2 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 ZrO 2 shows little impact on the electrochemical performance in comparison with the post-added ZrO 2 . With the substitution of Zr 4+ for Fe 3+ , the lattice parameter a decreases but c increases, resulting in the reduced lattice volume. In comparison with the Zr-free sample, Na 0.75 Mn 0.55 Ni 0.25 Co 0.05 Fe 0.10 Zr 0.05 O 2 (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 Zr 4+ can be summarized as follows: (1) Zr-O ( △H f 298K (Zr-O)   =   760   kJ   mol −1 ) has a stronger bond energy than that of Fe O ( △H f 298K (Fe-O)   =   409   kJ   mol −1 ), which enhances the structure stability and cycling performance; (2) the aliovalent substitution of Zr 4+ for Fe 3+ enhances the electronic conductivity, which is favorable for the rate capability; (3) the substitution of Zr 4+ for Fe 3+ 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.